attributes -> properties + orth centre check method

mdopt/mps/canonical.py

@@ -1,6 +1,6 @@
 """
 This module contains the :class:`CanonicalMPS` class.
-Hereafter, saying the MPS is in a canonical form will mean one of the following.
+Hereafter, saying a MPS is in a canonical form will mean one of the following.
 
     1) Right-canonical: all tensors are right isometries, i.e.::
 
@@ -21,12 +21,22 @@
     3) Mixed-canonical: all but one tensors are left or right isometries.
     This exceptional tensor will be hereafter called the **orthogonality centre**.
 
-    Note that, in the diagrams, a tensor with a star inside means that it is complex-conjugated.
+    Note, that in the diagrams, a tensor with a star inside means that it is complex-conjugated.
 
-    The Matrix Product State is stored as a list of three-dimensional tensors.
-    Essentially, it corresponds to storing each ``A[i]`` or ``B[i]`` as shown in
+    A Matrix Product State is thus stored as a list of three-dimensional tensors
+    as shown in the following diagram::
+
+               i    i+1
+        ...---( )---( )---...
+               |     |
+               |     |
+
+    Essentially, this corresponds to storing each ``A[i]`` or ``B[i]`` as shown in
     fig.4c in reference `[1]`_.
 
+    Note, that we enumerate the bonds from the right side of the tensors. For example,
+    the bond ``0`` is a bond to the right of tensor ``0``.
+
     .. _[1]: https://arxiv.org/abs/1805.00055
 """
 
@@ -37,7 +47,7 @@
 from opt_einsum import contract
 
 import mdopt
-from mdopt.utils.utils import kron_tensors, split_two_site_tensor
+from mdopt.utils.utils import svd, kron_tensors, split_two_site_tensor
 
 
 class CanonicalMPS:
@@ -64,7 +74,7 @@ class CanonicalMPS:
     num_sites : int
         Number of sites.
     num_bonds : int
-        Number of bonds.
+        Number of bonds. Note, that the "ghost" bonds are not included.
 
     Raises
     ------
@@ -84,10 +94,6 @@ def __init__(
         self.tensors = tensors
         self.num_sites = len(tensors)
         self.num_bonds = self.num_sites - 1
-        self.bond_dimensions = [
-            self.tensors[i].shape[-1] for i in range(self.num_bonds)
-        ]
-        self.phys_dimensions = [self.tensors[i].shape[1] for i in range(self.num_sites)]
         self.orth_centre = orth_centre
         self.dtype = tensors[0].dtype
         self.tolerance = tolerance
@@ -106,6 +112,20 @@ def __init__(
                     f"while the one at site {i} has {len(tensor.shape)}."
                 )
 
+    @property
+    def bond_dimensions(self) -> List[int]:
+        """
+        Returns the list of all bond dimensions of the MPS.
+        """
+        return [self.tensors[i].shape[-1] for i in range(self.num_bonds)]
+
+    @property
+    def phys_dimensions(self) -> List[int]:
+        """
+        Returns the list of all physical dimensions of the MPS.
+        """
+        return [self.tensors[i].shape[1] for i in range(self.num_sites)]
+
     def __len__(self) -> int:
         """
         Returns the number of sites in the MPS.
@@ -122,7 +142,7 @@ def copy(self) -> "CanonicalMPS":
 
     def reverse(self) -> "CanonicalMPS":
         """
-        Returns a reversed version of a given MPS.
+        Returns a reversed version of the current MPS.
         """
 
         reversed_tensors = [np.transpose(tensor) for tensor in reversed(self.tensors)]
@@ -301,6 +321,36 @@ def entanglement_entropy(
         """
         return self.explicit(tolerance=tolerance).entanglement_entropy()
 
+    def check_orth_centre(self) -> Optional[int]:
+        """
+        Checks the current position of the orthogonality centre by checking each tensor
+        for the isometry conditions.
+        Note, this method does not update the current instance's ``orth_centre`` attribute.
+        """
+
+        _, flags_left, flags_right = mdopt.mps.utils.find_orth_centre(  # type: ignore
+            self, return_orth_flags=True
+        )
+
+        if flags_left in (
+            [True] + [False] * (self.num_sites - 1),
+            [False] * self.num_sites,
+        ):
+            if flags_right == [not flag for flag in flags_left]:
+                return 0
+
+        if flags_left in (
+            [True] * (self.num_sites - 1) + [False],
+            [True] * self.num_sites,
+        ):
+            if flags_right == [not flag for flag in flags_left]:
+                return self.num_sites - 1
+
+        if all(flags_left) and all(flags_right):
+            return 0
+
+        return None
+
     def move_orth_centre(
         self,
         final_pos: int,
@@ -327,6 +377,8 @@ def move_orth_centre(
         ------
         ValueError
             If ``final_pos`` does not match the MPS length.
+        ValueError
+            If ``self.orth_centre`` is still ``None`` after the search.
         """
 
         if final_pos not in range(self.num_sites):
@@ -338,26 +390,7 @@ def move_orth_centre(
         singular_values = []
 
         if self.orth_centre is None:
-            _, flags_left, flags_right = mdopt.mps.utils.find_orth_centre(  # type: ignore
-                self, return_orth_flags=True
-            )
-
-            if flags_left in (
-                [True] + [False] * (self.num_sites - 1),
-                [False] * self.num_sites,
-            ):
-                if flags_right == [not flag for flag in flags_left]:
-                    self.orth_centre = 0
-
-            if flags_left in (
-                [True] * (self.num_sites - 1) + [False],
-                [True] * self.num_sites,
-            ):
-                if flags_right == [not flag for flag in flags_left]:
-                    self.orth_centre = self.num_sites - 1
-
-            if all(flags_left) and all(flags_right):
-                self.orth_centre = 0
+            self.orth_centre = self.check_orth_centre()
 
         if self.orth_centre is None:
             raise ValueError("The orthogonality centre value is set to None.")
@@ -380,6 +413,7 @@ def move_orth_centre(
                 two_site_tensor,
                 chi_max=self.chi_max,
                 renormalise=renormalise,
+                strategy="svd",
             )
             singular_values.append(singular_values_bond)
             mps.tensors[i] = u_l
@@ -411,31 +445,19 @@ def move_orth_centre_to_border(
         Returns a new version of the current :class:`CanonicalMPS` instance with
         the orthogonality centre moved to the closest (from the current position) border.
         """
-
         if self.orth_centre is None:
-            _, flags_left, flags_right = mdopt.mps.utils.find_orth_centre(  # type: ignore
-                self, return_orth_flags=True
-            )
-
-            if flags_left in (
-                [True] + [False] * (self.num_sites - 1),
-                [False] * self.num_sites,
-            ):
-                if flags_right == [not flag for flag in flags_left]:
-                    return self.copy(), "first"
+            self.orth_centre = self.check_orth_centre()
 
-            if flags_left in (
-                [True] * (self.num_sites - 1) + [False],
-                [True] * self.num_sites,
-            ):
-                if flags_right == [not flag for flag in flags_left]:
-                    return self.copy(), "last"
+        if self.orth_centre == 0:
+            return self.copy(), "first"
 
-            if all(flags_left) and all(flags_right):
-                return self.copy(), "first"
+        elif self.orth_centre == self.num_sites - 1:
+            return self.copy(), "last"
 
         else:
-            if self.orth_centre <= int(self.num_bonds / 2):
+            if (self.orth_centre is not None) and (
+                self.orth_centre <= int(self.num_bonds / 2)
+            ):
                 mps = self.move_orth_centre(
                     final_pos=0,
                     return_singular_values=False,
@@ -448,6 +470,7 @@ def move_orth_centre_to_border(
                 return_singular_values=False,
                 renormalise=renormalise,
             )
+
         return cast("CanonicalMPS", mps), "last"
 
     def explicit(