diff --git a/cytnx/UniTensor_conti.py b/cytnx/UniTensor_conti.py
index f8d8bdff..83e1f3c0 100644
--- a/cytnx/UniTensor_conti.py
+++ b/cytnx/UniTensor_conti.py
@@ -242,6 +242,6 @@ def at(self, labels:List[str], locator:List[int]):
 
 
 @add_method(UniTensor)
-def convert_from(self, Tin, force=False):
-    self.cfrom(Tin,force);
+def convert_from(self, Tin, force=False, tol = 1e-14):
+    self.cfrom(Tin,force,tol);
     return self
diff --git a/include/UniTensor.hpp b/include/UniTensor.hpp
index a8475a0e..6520545b 100644
--- a/include/UniTensor.hpp
+++ b/include/UniTensor.hpp
@@ -405,6 +405,8 @@ namespace cytnx {
     virtual const cytnx_int16 &at_for_sparse(const std::vector<cytnx_uint64> &locator,
                                              const cytnx_int16 &aux) const;
 
+    virtual void from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force,
+                       const cytnx_double &tol);
     virtual void from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force);
 
     virtual void group_basis_();
@@ -1700,7 +1702,8 @@ namespace cytnx {
         "This operation will destroy block structure. [Suggest] using get/set_block(s) to do "
         "operation on the block(s).");
     }
-    void from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force);
+    void from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force,
+               const cytnx_double &tol);
 
     void group_basis_();
 
@@ -4078,8 +4081,9 @@ namespace cytnx {
     void _Save(std::fstream &f) const;
     /// @endcond
 
-    UniTensor &convert_from(const UniTensor &rhs, const bool &force = false) {
-      this->_impl->from_(rhs._impl, force);
+    UniTensor &convert_from(const UniTensor &rhs, const bool &force = false,
+                            const cytnx_double &tol = 1e-14) {
+      this->_impl->from_(rhs._impl, force, tol);
       return *this;
     }
 
diff --git a/pybind/unitensor_py.cpp b/pybind/unitensor_py.cpp
index b77d385a..6a8839f1 100644
--- a/pybind/unitensor_py.cpp
+++ b/pybind/unitensor_py.cpp
@@ -1365,10 +1365,10 @@ void unitensor_binding(py::module &m) {
                 },
 				py::arg("low"), py::arg("high"), py::arg("seed")= -1)
 
-     .def("cfrom", [](UniTensor &self, const UniTensor &in, const bool &force){
-                         self.convert_from(in,force);
+     .def("cfrom", [](UniTensor &self, const UniTensor &in, const bool &force, const cytnx_double &tol){
+                         self.convert_from(in,force,tol);
                          },
-                    py::arg("Tin"), py::arg("force") = false)
+                    py::arg("Tin"), py::arg("force") = false, py::arg("tol") = 1e-14)
      .def("get_qindices",  [](UniTensor &self, const cytnx_uint64 &bidx){return self.get_qindices(bidx);});
   ;  // end of object line
 
diff --git a/src/BlockUniTensor.cpp b/src/BlockUniTensor.cpp
index a61fe5b6..df65be00 100644
--- a/src/BlockUniTensor.cpp
+++ b/src/BlockUniTensor.cpp
@@ -1992,7 +1992,8 @@ namespace cytnx {
     this->combineBonds(idx_mapper, force);
   }
 
-  void _BK_from_DN(BlockUniTensor *ths, DenseUniTensor *rhs, const bool &force) {
+  void _BK_from_DN(BlockUniTensor *ths, DenseUniTensor *rhs, const bool &force,
+                   const cytnx_double &tol) {
     if (!force) {
       // more checking:
       if (int(rhs->bond_(0).type()) != bondType::BD_NONE) {
@@ -2020,7 +2021,7 @@ namespace cytnx {
         elem = rhs->_block.at(cart);
       } else {
         if (!force)
-          if (abs(Scalar(rhs->_block.at(cart))) > 1e-14) {
+          if (abs(Scalar(rhs->_block.at(cart))) > tol) {
             cytnx_error_msg(true,
                             "[ERROR] force = false, trying to convert DenseUT to BlockUT that "
                             "violate the symmetry structure.%s",
@@ -2034,12 +2035,13 @@ namespace cytnx {
     cytnx_error_msg(true, "[ERROR] BlockUT-> BlockUT not implemented.%s", "\n");
   }
 
-  void BlockUniTensor::from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force) {
+  void BlockUniTensor::from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force,
+                             const cytnx_double &tol) {
     // checking shape:
     cytnx_error_msg(this->shape() != rhs->shape(), "[ERROR][from_] shape does not match.%s", "\n");
 
     if (rhs->uten_type() == UTenType.Dense) {
-      _BK_from_DN(this, (DenseUniTensor *)(rhs.get()), force);
+      _BK_from_DN(this, (DenseUniTensor *)(rhs.get()), force, tol);
     } else if (rhs->uten_type() == UTenType.Block) {
       _BK_from_BK(this, (BlockUniTensor *)(rhs.get()), force);
     } else {
diff --git a/src/UniTensor_base.cpp b/src/UniTensor_base.cpp
index 71f79457..6f30ab6a 100644
--- a/src/UniTensor_base.cpp
+++ b/src/UniTensor_base.cpp
@@ -699,7 +699,8 @@ namespace cytnx {
                     "\n");
   }
 
-  void UniTensor_base::from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force) {
+  void UniTensor_base::from_(const boost::intrusive_ptr<UniTensor_base> &rhs, const bool &force,
+                             const cytnx_double &tol) {
     cytnx_error_msg(true, "[ERROR] fatal internal, cannot call on a un-initialize UniTensor_base%s",
                     "\n");
   }