diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index ae370e4f..5ac49352 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -11,7 +11,7 @@ repos:
   - id: check-added-large-files
 
 - repo: https://github.com/pycqa/flake8
-  rev: 6.1.0
+  rev: 7.0.0
   hooks:
   - id: flake8
     args:
@@ -19,7 +19,7 @@ repos:
     - "--ignore=E203,W503"
 
 - repo: https://github.com/psf/black
-  rev: 23.12.0
+  rev: 24.1.1
   hooks:
   - id: black
     args:
diff --git a/doc/changelog.md b/doc/changelog.md
index 06ed8ba8..a2cfd955 100644
--- a/doc/changelog.md
+++ b/doc/changelog.md
@@ -2,6 +2,10 @@
 
 # Change Log
 
+## Feb-2-2024: Version 2.9.2
+
+- `boundary_mfp` value is stored in `kappa-*.hdf5` file when it is specified.
+
 ## Dec-26-2023: Version 2.9.1
 
 - Release to build conda-forge package.
diff --git a/doc/command-options.md b/doc/command-options.md
index 2e815fa0..a8b969d7 100644
--- a/doc/command-options.md
+++ b/doc/command-options.md
@@ -561,6 +561,8 @@ Then running with isotope calculation:
 In the result hdf5 file, currently isotope scattering strength is not written
 out, i.e., `gamma` is still imaginary part of self energy of ph-ph scattering.
 
+(boundary_mfp_option)=
+
 ### `--boundary-mfp`, `--bmfp` (`BOUNDARY_MFP`)
 
 A most simple phonon boundary scattering treatment is included. $v_g/L$ is just
diff --git a/doc/conf.py b/doc/conf.py
index b1aeed68..cae6e3a1 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -1,4 +1,5 @@
 """Sphinx phono3py configuration file."""
+
 #
 # phono3py documentation build configuration file, created by
 # sphinx-quickstart on Wed Jun 26 13:13:14 2013.
@@ -59,7 +60,7 @@
 # The short X.Y version.
 version = "2.9"
 # The full version, including alpha/beta/rc tags.
-release = "2.9.1"
+release = "2.9.2"
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
diff --git a/doc/hdf5_howto.md b/doc/hdf5_howto.md
index bf90878d..8569eac9 100644
--- a/doc/hdf5_howto.md
+++ b/doc/hdf5_howto.md
@@ -123,8 +123,9 @@ The array shape is (irreducible q-point, phonon band).
 (kappa_hdf5_file_gamma)=
 ### gamma
 
-Imaginary part of self energy. The physical unit is THz, where THz
-is in the ordinal frequency not the angular frequency.
+Imaginary part of self energy of phonon bubble diagram (phonon-phonon
+scattering). The physical unit is THz, where THz is in the ordinal frequency not
+the angular frequency.
 
 The array shape for all grid-points (irreducible q-points) is
 (temperature, irreducible q-point, phonon band).
@@ -262,9 +263,16 @@ P_{\mathbf{q}j} = \frac{1}{(3n_\mathrm{a})^2} \sum_{\lambda'\lambda''}
 $$
 
 This is not going to be calculated in the RTA thermal coductivity
-calculation mode by default. To calculate this, `--full_pp` option
+calculation mode by default. To calculate this, `--full-pp` option
 has to be specified (see {ref}`full_pp_option`).
 
+### boundary_mfp
+
+A value specified by {ref}`boundary_mfp_option`. The physical unit is
+micrometre.
+
+When `--boundary-mfp` option is explicitly specified, its value is stored here.
+
 ### kappa_unit_conversion
 
 This is used to convert the physical unit of lattice thermal
diff --git a/example/Si-PBEsol/Si.py b/example/Si-PBEsol/Si.py
index 763ddc8f..db2a8819 100644
--- a/example/Si-PBEsol/Si.py
+++ b/example/Si-PBEsol/Si.py
@@ -1,4 +1,5 @@
 """Example to run thermal conductivity of Si."""
+
 import numpy as np
 from phonopy.interface.vasp import read_vasp
 
diff --git a/example/zb-ZnTe-PBEsol/launch_phono3py_ZnTe_PBEsol_222.py b/example/zb-ZnTe-PBEsol/launch_phono3py_ZnTe_PBEsol_222.py
index 9ab45e18..064574c9 100644
--- a/example/zb-ZnTe-PBEsol/launch_phono3py_ZnTe_PBEsol_222.py
+++ b/example/zb-ZnTe-PBEsol/launch_phono3py_ZnTe_PBEsol_222.py
@@ -1,4 +1,5 @@
 """Launch script of ZnTe AiiDA calculation using aiida-phononpy."""
+
 from aiida.engine import submit
 from aiida.manage.configuration import load_profile
 from aiida.orm import Bool, Float, Str
diff --git a/phono3py/__init__.py b/phono3py/__init__.py
index d1e566cf..394c8838 100644
--- a/phono3py/__init__.py
+++ b/phono3py/__init__.py
@@ -1,4 +1,5 @@
 """Shortcuts to important classes, methods, and variables."""
+
 # Copyright (C) 2019 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/api_isotope.py b/phono3py/api_isotope.py
index 7bf4b6c9..de7b87eb 100644
--- a/phono3py/api_isotope.py
+++ b/phono3py/api_isotope.py
@@ -1,4 +1,5 @@
 """API for isotope scattering."""
+
 # Copyright (C) 2019 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/api_jointdos.py b/phono3py/api_jointdos.py
index a787e4a4..6b4841eb 100644
--- a/phono3py/api_jointdos.py
+++ b/phono3py/api_jointdos.py
@@ -1,4 +1,5 @@
 """API for joint-density-of-states calculation."""
+
 # Copyright (C) 2019 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/api_phono3py.py b/phono3py/api_phono3py.py
index dbb4d467..86efd8a7 100644
--- a/phono3py/api_phono3py.py
+++ b/phono3py/api_phono3py.py
@@ -1,4 +1,5 @@
 """Phono3py main class."""
+
 # Copyright (C) 2016 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/conductivity/base.py b/phono3py/conductivity/base.py
index 2123669d..05c14912 100644
--- a/phono3py/conductivity/base.py
+++ b/phono3py/conductivity/base.py
@@ -1,4 +1,5 @@
 """Thermal conductivity base class."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
@@ -375,6 +376,8 @@ def __init__(
         volume = self._pp.primitive.volume
         self._conversion_factor = unit_to_WmK / volume
 
+        self._averaged_pp_interaction = None
+
         # `self._velocity_obj` is the instance of an inherited class of
         # `GroupVelocity`. `self._init_velocity()` is the method setup the instance,
         # which must be implmented in the inherited class of `ConductivityBase`.
@@ -632,6 +635,11 @@ def get_averaged_pp_interaction(self):
         )
         return self.averaged_pp_interaction
 
+    @property
+    def boundary_mfp(self) -> float:
+        """Return boundary MFP."""
+        return self._boundary_mfp
+
     def get_number_of_sampling_grid_points(self):
         """Return number of grid points.
 
diff --git a/phono3py/conductivity/direct_solution.py b/phono3py/conductivity/direct_solution.py
index 132a4fd6..b8e027cb 100644
--- a/phono3py/conductivity/direct_solution.py
+++ b/phono3py/conductivity/direct_solution.py
@@ -1,4 +1,5 @@
 """Calculate lattice thermal conductivity by direct solution."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
@@ -108,7 +109,6 @@ def __init__(
         self._init_velocity(gv_delta_q)
 
         self._lang = lang
-        self._averaged_pp_interaction = None
         self._collision_eigenvalues = None
         self._is_reducible_collision_matrix = is_reducible_collision_matrix
         self._solve_collective_phonon = solve_collective_phonon
@@ -517,9 +517,9 @@ def _set_collision_matrix_at_sigmas(self, i_gp):
                 else:
                     i_data = 0
                 self._gamma[j, k, i_data] = self._collision.imag_self_energy
-                self._collision_matrix[
-                    j, k, i_data
-                ] = self._collision.get_collision_matrix()
+                self._collision_matrix[j, k, i_data] = (
+                    self._collision.get_collision_matrix()
+                )
 
     def _prepare_collision_matrix(self):
         """Collect pieces and construct collision matrix."""
diff --git a/phono3py/conductivity/kubo.py b/phono3py/conductivity/kubo.py
index 901b0eed..34da6283 100644
--- a/phono3py/conductivity/kubo.py
+++ b/phono3py/conductivity/kubo.py
@@ -1,4 +1,5 @@
 """Kubo thermal conductivity base class."""
+
 # Copyright (C) 2022 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/conductivity/rta.py b/phono3py/conductivity/rta.py
index be3fd1f0..64ac8bbe 100644
--- a/phono3py/conductivity/rta.py
+++ b/phono3py/conductivity/rta.py
@@ -1,4 +1,5 @@
 """Lattice thermal conductivity calculation with RTA."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
@@ -97,7 +98,6 @@ def __init__(
         self._gamma_detail_at_q = None
         self._use_ave_pp = use_ave_pp
         self._use_const_ave_pp = None
-        self._averaged_pp_interaction = None
         self._num_ignored_phonon_modes = None
 
         super().__init__(
@@ -298,9 +298,9 @@ def _set_gamma_at_sigmas(self, i):
                     self._gamma_N[j, k, i] = g_N
                     self._gamma_U[j, k, i] = g_U
                 if self._is_gamma_detail:
-                    self._gamma_detail_at_q[
-                        k
-                    ] = self._collision.get_detailed_imag_self_energy()
+                    self._gamma_detail_at_q[k] = (
+                        self._collision.get_detailed_imag_self_energy()
+                    )
 
     def _set_gamma_at_sigmas_lowmem(self, i):
         """Calculate gamma without storing ph-ph interaction strength.
diff --git a/phono3py/conductivity/utils.py b/phono3py/conductivity/utils.py
index 3059c2d2..62b62801 100644
--- a/phono3py/conductivity/utils.py
+++ b/phono3py/conductivity/utils.py
@@ -1,4 +1,5 @@
 """Utilities for lattice thermal conductivity calculation."""
+
 # Copyright (C) 2022 Atsushi Togo
 # All rights reserved.
 #
@@ -268,6 +269,7 @@ def write_kappa(
         qpoints = br.qpoints
         grid_points = br.grid_points
         weights = br.grid_weights
+        boundary_mfp = br.boundary_mfp
 
         for i, sigma in enumerate(sigmas):
             if kappa is None:
@@ -314,6 +316,7 @@ def write_kappa(
             write_kappa_to_hdf5(
                 temperatures,
                 mesh,
+                boundary_mfp=boundary_mfp,
                 bz_grid=bz_grid,
                 frequency=frequencies,
                 group_velocity=gv,
@@ -487,13 +490,13 @@ def write_collision(
     @staticmethod
     def write_kappa(
         lbte: "cond_LBTE_type",
-        volume,
-        is_reducible_collision_matrix=False,
-        write_LBTE_solution=False,
-        pinv_solver=None,
-        compression="gzip",
-        filename=None,
-        log_level=0,
+        volume: float,
+        is_reducible_collision_matrix: bool = False,
+        write_LBTE_solution: bool = False,
+        pinv_solver: Optional[int] = None,
+        compression: str = "gzip",
+        filename: Optional[str] = None,
+        log_level: int = 0,
     ):
         """Write kappa related properties into a hdf5 file."""
         from phono3py.conductivity.direct_solution import (
@@ -546,6 +549,7 @@ def write_kappa(
         f_vector = lbte.get_f_vectors()
         mode_cv = lbte.mode_heat_capacities
         mfp = lbte.get_mean_free_path()
+        boundary_mfp = lbte.boundary_mfp
 
         coleigs = lbte.get_collision_eigenvalues()
         # After kappa calculation, the variable is overwritten by unitary matrix
@@ -612,6 +616,7 @@ def write_kappa(
             write_kappa_to_hdf5(
                 temperatures,
                 mesh,
+                boundary_mfp=boundary_mfp,
                 bz_grid=bz_grid,
                 frequency=frequencies,
                 group_velocity=gv,
diff --git a/phono3py/conductivity/wigner.py b/phono3py/conductivity/wigner.py
index a6d754d0..9f91b609 100644
--- a/phono3py/conductivity/wigner.py
+++ b/phono3py/conductivity/wigner.py
@@ -1,4 +1,5 @@
 """Wigner thermal conductivity base class."""
+
 # Copyright (C) 2022 Michele Simoncelli
 # All rights reserved.
 #
diff --git a/phono3py/cui/create_force_constants.py b/phono3py/cui/create_force_constants.py
index feb9873e..d0042586 100644
--- a/phono3py/cui/create_force_constants.py
+++ b/phono3py/cui/create_force_constants.py
@@ -1,4 +1,5 @@
 """Force constants calculation utilities for command line user interface."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/create_supercells.py b/phono3py/cui/create_supercells.py
index 0ff27a8f..55977358 100644
--- a/phono3py/cui/create_supercells.py
+++ b/phono3py/cui/create_supercells.py
@@ -1,4 +1,5 @@
 """Utilities of main CUI script."""
+
 # Copyright (C) 2015 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/kaccum_script.py b/phono3py/cui/kaccum_script.py
index 598267be..744f8bbe 100644
--- a/phono3py/cui/kaccum_script.py
+++ b/phono3py/cui/kaccum_script.py
@@ -1,4 +1,5 @@
 """Phono3py kaccum command line script."""
+
 import argparse
 import sys
 
diff --git a/phono3py/cui/load.py b/phono3py/cui/load.py
index 04a42847..59456ed6 100644
--- a/phono3py/cui/load.py
+++ b/phono3py/cui/load.py
@@ -1,4 +1,5 @@
 """Phono3py loader."""
+
 # Copyright (C) 2019 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/phono3py_argparse.py b/phono3py/cui/phono3py_argparse.py
index 0c24ba78..0cbdc115 100644
--- a/phono3py/cui/phono3py_argparse.py
+++ b/phono3py/cui/phono3py_argparse.py
@@ -1,4 +1,5 @@
 """Phono3py command option argument parser."""
+
 # Copyright (C) 2015 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/phono3py_script.py b/phono3py/cui/phono3py_script.py
index 8880f076..86ffeba2 100644
--- a/phono3py/cui/phono3py_script.py
+++ b/phono3py/cui/phono3py_script.py
@@ -1,4 +1,5 @@
 """Phono3py main command line script."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/settings.py b/phono3py/cui/settings.py
index 960fb524..5f35b37f 100644
--- a/phono3py/cui/settings.py
+++ b/phono3py/cui/settings.py
@@ -1,4 +1,5 @@
 """Phono3py command option and conf file parser."""
+
 # Copyright (C) 2015 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/show_log.py b/phono3py/cui/show_log.py
index 7d37c9c8..2283e065 100644
--- a/phono3py/cui/show_log.py
+++ b/phono3py/cui/show_log.py
@@ -1,4 +1,5 @@
 """Utilities to show various logs for main CUI script."""
+
 # Copyright (C) 2015 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/cui/triplets_info.py b/phono3py/cui/triplets_info.py
index 9e2ae990..50ced6a5 100644
--- a/phono3py/cui/triplets_info.py
+++ b/phono3py/cui/triplets_info.py
@@ -1,4 +1,5 @@
 """Show and write triplets information."""
+
 # Copyright (C) 2015 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/file_IO.py b/phono3py/file_IO.py
index 7856a4d9..20040997 100644
--- a/phono3py/file_IO.py
+++ b/phono3py/file_IO.py
@@ -1,4 +1,5 @@
 """File I/O methods."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
@@ -32,9 +33,12 @@
 # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
+from __future__ import annotations
 
 import os
 import warnings
+from collections.abc import Sequence
+from typing import Optional, Union
 
 import h5py
 import numpy as np
@@ -918,6 +922,7 @@ def write_collision_eigenvalues_to_hdf5(
 def write_kappa_to_hdf5(
     temperature,
     mesh,
+    boundary_mfp: float = None,
     bz_grid=None,
     frequency=None,
     group_velocity=None,
@@ -1058,6 +1063,8 @@ def write_kappa_to_hdf5(
             w.create_dataset("sigma_cutoff_width", data=sigma_cutoff)
         if kappa_unit_conversion is not None:
             w.create_dataset("kappa_unit_conversion", data=kappa_unit_conversion)
+        if boundary_mfp is not None:
+            w.create_dataset("boundary_mfp", data=boundary_mfp)
 
         if verbose:
             text = "Thermal conductivity related properties "
@@ -1720,12 +1727,12 @@ def get_length_of_first_line(f):
 
 def _get_filename_suffix(
     mesh,
-    grid_point=None,
-    band_indices=None,
-    sigma=None,
-    sigma_cutoff=None,
-    temperature=None,
-    filename=None,
+    grid_point: Optional[int] = None,
+    band_indices: Optional[Union[np.ndarray, Sequence]] = None,
+    sigma: Optional[float] = None,
+    sigma_cutoff: Optional[float] = None,
+    temperature: Optional[float] = None,
+    filename: Optional[str] = None,
 ):
     """Return filename suffix corresponding to parameters."""
     suffix = "-m%d%d%d" % tuple(mesh)
diff --git a/phono3py/interface/alm.py b/phono3py/interface/alm.py
index 804b8702..91292761 100644
--- a/phono3py/interface/alm.py
+++ b/phono3py/interface/alm.py
@@ -1,4 +1,5 @@
 """ALM interface for force constants calculation."""
+
 # Copyright (C) 2016 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/interface/calculator.py b/phono3py/interface/calculator.py
index 1a987277..15f0014d 100644
--- a/phono3py/interface/calculator.py
+++ b/phono3py/interface/calculator.py
@@ -1,4 +1,5 @@
 """Utilities of calculator interfaces."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/interface/fc_calculator.py b/phono3py/interface/fc_calculator.py
index c80cf1ce..239f1fc0 100644
--- a/phono3py/interface/fc_calculator.py
+++ b/phono3py/interface/fc_calculator.py
@@ -1,4 +1,5 @@
 """Interfaces for force constants calculators."""
+
 # Copyright (C) 2019 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/interface/phono3py_yaml.py b/phono3py/interface/phono3py_yaml.py
index f2ed69f8..79c03256 100644
--- a/phono3py/interface/phono3py_yaml.py
+++ b/phono3py/interface/phono3py_yaml.py
@@ -1,4 +1,5 @@
 """phono3py_yaml reader and writer."""
+
 # Copyright (C) 2016 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/other/isotope.py b/phono3py/other/isotope.py
index 8f1bacfc..a786c08f 100644
--- a/phono3py/other/isotope.py
+++ b/phono3py/other/isotope.py
@@ -1,4 +1,5 @@
 """Isotope scattering calculation."""
+
 # Copyright (C) 2015 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/other/kaccum.py b/phono3py/other/kaccum.py
index f95b3651..e57b1cbf 100644
--- a/phono3py/other/kaccum.py
+++ b/phono3py/other/kaccum.py
@@ -1,4 +1,5 @@
 """Calculated accumulated property with respect to other property."""
+
 from typing import Optional
 
 import numpy as np
diff --git a/phono3py/other/tetrahedron_method.py b/phono3py/other/tetrahedron_method.py
index bbe34198..83a280fc 100644
--- a/phono3py/other/tetrahedron_method.py
+++ b/phono3py/other/tetrahedron_method.py
@@ -1,4 +1,5 @@
 """Tetrahedron method python wrapper."""
+
 # Copyright (C) 2021 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon/func.py b/phono3py/phonon/func.py
index e320baa5..434feefd 100644
--- a/phono3py/phonon/func.py
+++ b/phono3py/phonon/func.py
@@ -1,4 +1,5 @@
 """Mathematical functions."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon/grid.py b/phono3py/phonon/grid.py
index d36457ec..ea7bc30f 100644
--- a/phono3py/phonon/grid.py
+++ b/phono3py/phonon/grid.py
@@ -1,4 +1,5 @@
 """Regular grid tools."""
+
 # Copyright (C) 2021 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon/group_velocity_matrix.py b/phono3py/phonon/group_velocity_matrix.py
index 8f0017d7..b877875d 100644
--- a/phono3py/phonon/group_velocity_matrix.py
+++ b/phono3py/phonon/group_velocity_matrix.py
@@ -1,4 +1,5 @@
 """Calculate group velocity matrix."""
+
 # Copyright (C) 2021 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon/heat_capacity_matrix.py b/phono3py/phonon/heat_capacity_matrix.py
index 8b4ef6d8..05056de6 100644
--- a/phono3py/phonon/heat_capacity_matrix.py
+++ b/phono3py/phonon/heat_capacity_matrix.py
@@ -1,4 +1,5 @@
 """Calculate group velocity matrix."""
+
 # Copyright (C) 2021 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon/solver.py b/phono3py/phonon/solver.py
index 6307b056..983ee81e 100644
--- a/phono3py/phonon/solver.py
+++ b/phono3py/phonon/solver.py
@@ -1,4 +1,5 @@
 """Create dynamical matrix and solve harmonic phonons on grid."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon/velocity_operator.py b/phono3py/phonon/velocity_operator.py
index b1f68723..bc7ca02c 100644
--- a/phono3py/phonon/velocity_operator.py
+++ b/phono3py/phonon/velocity_operator.py
@@ -1,4 +1,5 @@
 """Velocity operator of Simoncelli, Marzari, and Mauri."""
+
 # Copyright (C) 2013 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/collision_matrix.py b/phono3py/phonon3/collision_matrix.py
index 609085fa..a7fd3764 100644
--- a/phono3py/phonon3/collision_matrix.py
+++ b/phono3py/phonon3/collision_matrix.py
@@ -1,4 +1,5 @@
 """Calculate collision matrix of direct solution of LBTE."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/dataset.py b/phono3py/phonon3/dataset.py
index e896056b..b6e11663 100644
--- a/phono3py/phonon3/dataset.py
+++ b/phono3py/phonon3/dataset.py
@@ -1,4 +1,5 @@
 """Parse displacement dataset."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/displacement_fc3.py b/phono3py/phonon3/displacement_fc3.py
index f430c099..4f539977 100644
--- a/phono3py/phonon3/displacement_fc3.py
+++ b/phono3py/phonon3/displacement_fc3.py
@@ -1,4 +1,5 @@
 """Procedures to handle atomic displacements for fc3."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/fc3.py b/phono3py/phonon3/fc3.py
index 6b23e6bf..63e1e6c2 100644
--- a/phono3py/phonon3/fc3.py
+++ b/phono3py/phonon3/fc3.py
@@ -1,4 +1,5 @@
 """Calculate fc3."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/gruneisen.py b/phono3py/phonon3/gruneisen.py
index 8083d3c0..c803d22b 100644
--- a/phono3py/phonon3/gruneisen.py
+++ b/phono3py/phonon3/gruneisen.py
@@ -1,4 +1,5 @@
 """Calculation of mode Grueneisen parameters from fc3."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/imag_self_energy.py b/phono3py/phonon3/imag_self_energy.py
index 39b13ff9..563ba6a6 100644
--- a/phono3py/phonon3/imag_self_energy.py
+++ b/phono3py/phonon3/imag_self_energy.py
@@ -1,4 +1,5 @@
 """Calculation of imaginary-part of self-energy of bubble diagram."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/interaction.py b/phono3py/phonon3/interaction.py
index 4015aa50..581250d9 100644
--- a/phono3py/phonon3/interaction.py
+++ b/phono3py/phonon3/interaction.py
@@ -1,4 +1,5 @@
 """Calculate ph-ph interaction and phonons on grid."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/joint_dos.py b/phono3py/phonon3/joint_dos.py
index e183fb91..69fe9007 100644
--- a/phono3py/phonon3/joint_dos.py
+++ b/phono3py/phonon3/joint_dos.py
@@ -1,4 +1,5 @@
 """Joint-density of states calculation."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/real_self_energy.py b/phono3py/phonon3/real_self_energy.py
index c1e026fd..a49ee932 100644
--- a/phono3py/phonon3/real_self_energy.py
+++ b/phono3py/phonon3/real_self_energy.py
@@ -1,4 +1,5 @@
 """Calculate real-part of self-energy of bubble diagram."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/real_to_reciprocal.py b/phono3py/phonon3/real_to_reciprocal.py
index aca2145c..cafa700a 100644
--- a/phono3py/phonon3/real_to_reciprocal.py
+++ b/phono3py/phonon3/real_to_reciprocal.py
@@ -1,4 +1,5 @@
 """Transform fc3 in real space to reciprocal space."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/reciprocal_to_normal.py b/phono3py/phonon3/reciprocal_to_normal.py
index 0d569068..9cf2970e 100644
--- a/phono3py/phonon3/reciprocal_to_normal.py
+++ b/phono3py/phonon3/reciprocal_to_normal.py
@@ -1,4 +1,5 @@
 """Transform fc3 in reciprocal space to phonon space."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/spectral_function.py b/phono3py/phonon3/spectral_function.py
index 9b6943d1..4b0b9f99 100644
--- a/phono3py/phonon3/spectral_function.py
+++ b/phono3py/phonon3/spectral_function.py
@@ -1,4 +1,5 @@
 """Calculate spectral function due to bubble diagram."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/phonon3/triplets.py b/phono3py/phonon3/triplets.py
index e6d9b755..139240a6 100644
--- a/phono3py/phonon3/triplets.py
+++ b/phono3py/phonon3/triplets.py
@@ -1,4 +1,5 @@
 """Utilities to handle q-point triplets."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
diff --git a/phono3py/version.py b/phono3py/version.py
index 14d42a0d..434dd992 100644
--- a/phono3py/version.py
+++ b/phono3py/version.py
@@ -1,4 +1,5 @@
 """Define phono3py version."""
+
 # Copyright (C) 2020 Atsushi Togo
 # All rights reserved.
 #
@@ -33,4 +34,4 @@
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
-__version__ = "2.9.1"
+__version__ = "2.9.2"
diff --git a/setup.py b/setup.py
index f1cd0468..7f0c1780 100644
--- a/setup.py
+++ b/setup.py
@@ -7,6 +7,7 @@
 set PHONO3PY_USE_CMAKE=false to avoid running cmake.
 
 """
+
 import os
 import pathlib
 import shutil
diff --git a/test/api/test_api_phono3py.py b/test/api/test_api_phono3py.py
index 302e1e7c..38687170 100644
--- a/test/api/test_api_phono3py.py
+++ b/test/api/test_api_phono3py.py
@@ -1,4 +1,5 @@
 """Tests of Phono3py API."""
+
 from pathlib import Path
 
 from phono3py import Phono3py
diff --git a/test/conductivity/test_kappa_LBTE.py b/test/conductivity/test_kappa_LBTE.py
index b7715791..f494b0e0 100644
--- a/test/conductivity/test_kappa_LBTE.py
+++ b/test/conductivity/test_kappa_LBTE.py
@@ -1,4 +1,5 @@
 """Tests for direct solution of LBTE."""
+
 import numpy as np
 
 from phono3py.api_phono3py import Phono3py
diff --git a/test/conductivity/test_kappa_LBTE_Wigner.py b/test/conductivity/test_kappa_LBTE_Wigner.py
index f4b805fe..9f6cb548 100644
--- a/test/conductivity/test_kappa_LBTE_Wigner.py
+++ b/test/conductivity/test_kappa_LBTE_Wigner.py
@@ -1,4 +1,5 @@
 """Tests for direct solution of LBTE."""
+
 import numpy as np
 import pytest
 
diff --git a/test/conductivity/test_kappa_RTA.py b/test/conductivity/test_kappa_RTA.py
index 499a64e1..46643819 100644
--- a/test/conductivity/test_kappa_RTA.py
+++ b/test/conductivity/test_kappa_RTA.py
@@ -1,4 +1,5 @@
 """Test for Conductivity_RTA.py."""
+
 import itertools
 
 import numpy as np
diff --git a/test/conductivity/test_kappa_RTA_Wigner.py b/test/conductivity/test_kappa_RTA_Wigner.py
index eb1020ed..37426406 100644
--- a/test/conductivity/test_kappa_RTA_Wigner.py
+++ b/test/conductivity/test_kappa_RTA_Wigner.py
@@ -1,4 +1,5 @@
 """Test for Conductivity_RTA.py."""
+
 import itertools
 
 import numpy as np
diff --git a/test/conftest.py b/test/conftest.py
index 3bb8515d..bfb10096 100644
--- a/test/conftest.py
+++ b/test/conftest.py
@@ -1,4 +1,5 @@
 """Pytest conftest.py."""
+
 from pathlib import Path
 
 import numpy as np
diff --git a/test/cui/test_phono3py_load_script.py b/test/cui/test_phono3py_load_script.py
index 3540e274..e80627dd 100644
--- a/test/cui/test_phono3py_load_script.py
+++ b/test/cui/test_phono3py_load_script.py
@@ -1,4 +1,5 @@
 """Tests of Phono3py API."""
+
 from __future__ import annotations
 
 import os
diff --git a/test/file_IO/test_file_IO.py b/test/file_IO/test_file_IO.py
new file mode 100644
index 00000000..a6da0028
--- /dev/null
+++ b/test/file_IO/test_file_IO.py
@@ -0,0 +1,95 @@
+"""Tests of Phono3py API."""
+
+import pathlib
+from collections.abc import Sequence
+from typing import Optional
+
+import h5py
+import numpy as np
+
+from phono3py import Phono3py
+from phono3py.file_IO import _get_filename_suffix
+
+cwd = pathlib.Path(__file__).parent
+cwd_called = pathlib.Path.cwd()
+
+
+def test_kappa_filename():
+    """Test _get_filename_suffix."""
+    mesh = [4, 4, 4]
+    grid_point = None
+    band_indices = None
+    sigma = None
+    sigma_cutoff = None
+    filename = None
+    suffix = _get_filename_suffix(
+        mesh,
+        grid_point=grid_point,
+        band_indices=band_indices,
+        sigma=sigma,
+        sigma_cutoff=sigma_cutoff,
+        filename=filename,
+    )
+    full_filename = "kappa" + suffix + ".hdf5"
+    assert full_filename == "kappa-m444.hdf5"
+
+
+def test_kappa_hdf5_with_boundary_mpf(si_pbesol: Phono3py):
+    """Test boundary_mfp in kappa-*.hdf5.
+
+    Remember to clean files created by
+    Phono3py.run_thermal_conductivity(write_kappa=True).
+
+    """
+    key_ref = [
+        "boundary_mfp",
+        "frequency",
+        "gamma",
+        "grid_point",
+        "group_velocity",
+        "gv_by_gv",
+        "heat_capacity",
+        "kappa",
+        "kappa_unit_conversion",
+        "mesh",
+        "mode_kappa",
+        "qpoint",
+        "temperature",
+        "version",
+        "weight",
+    ]
+
+    boundary_mfp = 10000.0
+    kappa_filename = _set_kappa(
+        si_pbesol, [4, 4, 4], write_kappa=True, boundary_mfp=boundary_mfp
+    )
+    file_path = pathlib.Path(cwd_called / kappa_filename)
+    with h5py.File(file_path, "r") as f:
+        np.testing.assert_almost_equal(f["boundary_mfp"][()], boundary_mfp)
+        assert set(list(f)) == set(key_ref)
+
+    if file_path.exists():
+        file_path.unlink()
+
+
+def _set_kappa(
+    ph3: Phono3py,
+    mesh: Sequence,
+    is_isotope: bool = False,
+    is_full_pp: bool = False,
+    write_kappa: bool = False,
+    boundary_mfp: Optional[float] = None,
+) -> str:
+    ph3.mesh_numbers = mesh
+    ph3.init_phph_interaction()
+    ph3.run_thermal_conductivity(
+        temperatures=[
+            300,
+        ],
+        is_isotope=is_isotope,
+        is_full_pp=is_full_pp,
+        write_kappa=write_kappa,
+        boundary_mfp=boundary_mfp,
+    )
+    suffix = _get_filename_suffix(mesh)
+    return "kappa" + suffix + ".hdf5"
diff --git a/test/interface/test_phono3py_yaml.py b/test/interface/test_phono3py_yaml.py
index 954f6abb..6702c50f 100644
--- a/test/interface/test_phono3py_yaml.py
+++ b/test/interface/test_phono3py_yaml.py
@@ -1,4 +1,5 @@
 """Tests of PhonopyYaml."""
+
 from io import StringIO
 from pathlib import Path
 
diff --git a/test/other/test_isotope.py b/test/other/test_isotope.py
index 01594fa8..3e168326 100644
--- a/test/other/test_isotope.py
+++ b/test/other/test_isotope.py
@@ -1,4 +1,5 @@
 """Tests for isotope scatterings."""
+
 import numpy as np
 import pytest
 
diff --git a/test/other/test_kaccum.py b/test/other/test_kaccum.py
index a2149dd4..47cf5b86 100644
--- a/test/other/test_kaccum.py
+++ b/test/other/test_kaccum.py
@@ -1,4 +1,5 @@
 """Test for kaccum.py."""
+
 import numpy as np
 
 from phono3py import Phono3py
diff --git a/test/phonon/test_grid.py b/test/phonon/test_grid.py
index 0e187d6e..23905353 100644
--- a/test/phonon/test_grid.py
+++ b/test/phonon/test_grid.py
@@ -1,4 +1,5 @@
 """Tests for grids."""
+
 from __future__ import annotations
 
 import numpy as np
diff --git a/test/phonon/test_velocity_operator.py b/test/phonon/test_velocity_operator.py
index 55a17680..1a51e700 100644
--- a/test/phonon/test_velocity_operator.py
+++ b/test/phonon/test_velocity_operator.py
@@ -1,4 +1,5 @@
 """Tests for velocity operator calculation."""
+
 import numpy as np
 from phonopy import Phonopy
 from phonopy.units import THzToCm, VaspToTHz
diff --git a/test/phonon3/test_displacements.py b/test/phonon3/test_displacements.py
index 978f0222..992772e0 100644
--- a/test/phonon3/test_displacements.py
+++ b/test/phonon3/test_displacements.py
@@ -1,4 +1,5 @@
 """Tests of displacements.py."""
+
 import numpy as np
 
 import phono3py
diff --git a/test/phonon3/test_fc3.py b/test/phonon3/test_fc3.py
index 1dab5060..65ba7533 100644
--- a/test/phonon3/test_fc3.py
+++ b/test/phonon3/test_fc3.py
@@ -1,4 +1,5 @@
 """Tests for fc3."""
+
 import numpy as np
 import pytest
 
diff --git a/test/phonon3/test_imag_self_energy.py b/test/phonon3/test_imag_self_energy.py
index edae5771..9dfca8f6 100644
--- a/test/phonon3/test_imag_self_energy.py
+++ b/test/phonon3/test_imag_self_energy.py
@@ -1,4 +1,5 @@
 """Test for imag_free_energy.py."""
+
 import numpy as np
 import pytest
 
diff --git a/test/phonon3/test_interaction.py b/test/phonon3/test_interaction.py
index 06da2b3b..29696fa2 100644
--- a/test/phonon3/test_interaction.py
+++ b/test/phonon3/test_interaction.py
@@ -1,4 +1,5 @@
 """Test Interaction class."""
+
 from __future__ import annotations
 
 from collections.abc import Sequence
diff --git a/test/phonon3/test_joint_dos.py b/test/phonon3/test_joint_dos.py
index eca9a971..cb48cb31 100644
--- a/test/phonon3/test_joint_dos.py
+++ b/test/phonon3/test_joint_dos.py
@@ -1,4 +1,5 @@
 """Tests for joint-density-of-states."""
+
 import numpy as np
 import pytest
 
diff --git a/test/phonon3/test_real_self_energy.py b/test/phonon3/test_real_self_energy.py
index c51c37d9..7eadc599 100644
--- a/test/phonon3/test_real_self_energy.py
+++ b/test/phonon3/test_real_self_energy.py
@@ -1,4 +1,5 @@
 """Test for real_self_energy.py."""
+
 import numpy as np
 
 from phono3py import Phono3py
diff --git a/test/phonon3/test_spectral_function.py b/test/phonon3/test_spectral_function.py
index 947c2cee..6a25630b 100644
--- a/test/phonon3/test_spectral_function.py
+++ b/test/phonon3/test_spectral_function.py
@@ -1,4 +1,5 @@
 """Test spectral_function.py."""
+
 import numpy as np
 import pytest
 
diff --git a/test/phonon3/test_triplets.py b/test/phonon3/test_triplets.py
index 9fd0d8cc..dceb62a8 100644
--- a/test/phonon3/test_triplets.py
+++ b/test/phonon3/test_triplets.py
@@ -1,4 +1,5 @@
 """Test for triplets.py."""
+
 import numpy as np
 import pytest
 from phonopy import Phonopy
diff --git a/test/sscha/test_sscha.py b/test/sscha/test_sscha.py
index b0839bda..88388cc9 100644
--- a/test/sscha/test_sscha.py
+++ b/test/sscha/test_sscha.py
@@ -1,4 +1,5 @@
 """Tests for SSCHA routines."""
+
 import numpy as np
 import pytest
 from phonopy.phonon.qpoints import QpointsPhonon