use the mock to avoid installation in sphinx

doc/conf.py

@@ -13,8 +13,19 @@
 #
 import os
 import sys
+from unittest.mock import MagicMock
 
 sys.path.insert(0, os.path.abspath("../"))
+# List of modules to mock
+MOCK_MODULES = ['torchani', 'lammps']
+
+class Mock(MagicMock):
+    @classmethod
+    def __getattr__(cls, name):
+        return MagicMock()
+
+sys.modules.update((mod_name, Mock()) for mod_name in MOCK_MODULES)
+
 
 # -- Project information -----------------------------------------------------
 

pyxtal/interface/lammpslib.py

@@ -9,10 +9,7 @@
 from ase.constraints import UnitCellFilter, FixSymmetry
 from ase.optimize import LBFGS
 from ase.optimize.fire import FIRE
-try:
-    from lammps import lammps
-except:
-    raise NotImplementedError('PLZ INSTALL lammps first')
+from lammps import lammps
 
 
 #