In case a generally contracted shell can be split into independent bl…

…ocks, do so.

basis_set_exchange/api.py

@@ -232,6 +232,9 @@ def get_basis(name,
         basis_dict = manip.optimize_general(basis_dict, False)
         needs_pruning = True
 
+    # Split any blocked contractions
+    basis_dict = manip.split_blocked_contractions(basis_dict, False)
+
     # uncontract_segmented implies uncontract_general
     if uncontract_segmented:
         basis_dict = manip.uncontract_segmented(basis_dict, False)

basis_set_exchange/manip.py

@@ -763,3 +763,99 @@ def truhlar_calendarize(basis, month, use_copy=True):
 
     basis = prune_basis(basis, False)
     return basis
+
+
+def split_blocked_contractions(basis, use_copy=True):
+    '''Checks if the contraction coefficients in a general contraction can be made block diagonal and thereby split into two or more distinct shells
+
+
+    Parameters
+    ----------
+    basis : dict
+        Basis set dictionary to work with
+    use_copy: bool
+        If True, the input basis set is not modified.
+    '''
+
+    if use_copy:
+        basis = copy.deepcopy(basis)
+
+    for eldata in basis['elements'].values():
+
+        if 'electron_shells' not in eldata:
+            continue
+
+        orig_shells = eldata['electron_shells']
+        new_shells = []
+        for sh in orig_shells:
+            coefficients = sh['coefficients']
+            ncontr = len(coefficients)
+            nam = len(sh['angular_momentum'])
+            # Skip sp shells and shells with only one general contraction
+            if nam > 1 or ncontr == 1:
+                new_shells.append(sh)
+                continue
+
+            exponents = sh['exponents']
+            nprim = len(exponents)
+
+            # Figure out which contractions share primitives between them
+            shared_primitives = [[False for _ in range(ncontr)] for _ in range(ncontr)]
+            for icontr in range(ncontr):
+                for jcontr in range(icontr + 1):
+                    for iprim in range(nprim):
+                        if float(coefficients[icontr][iprim]) != 0.0 and float(coefficients[jcontr][iprim]) != 0.0:
+                            shared_primitives[icontr][jcontr] = True
+                            shared_primitives[jcontr][icontr] = True
+                            break
+
+            # Which contractions have been processed
+            contraction_processed = [False for _ in range(ncontr)]
+
+            # Indices of the contractions that are coupled
+            blocks = []
+            for icontr in range(ncontr):
+                if not contraction_processed[icontr]:
+                    # List of contracted functions in the block
+                    block = [icontr]
+                    contraction_processed[icontr] = True
+
+                    # Form the list
+                    for jcontr in range(icontr + 1, ncontr):
+                        # No need to analyze functions that have already been processed
+                        if contraction_processed[jcontr]:
+                            continue
+                        if shared_primitives[icontr][jcontr]:
+                            block.append(jcontr)
+                            contraction_processed[jcontr] = True
+                    blocks.append(block)
+
+            # Do we need to do anything?
+            if len(blocks) == 1:
+                # All functions are in a single block; we keep the shell as it is
+                new_shells.append(sh)
+                continue
+
+            # Create new shells
+            for block in blocks:
+                # Identify the used primitives
+                used_primitives = []
+                for iprim in range(nprim):
+                    for icontr in block:
+                        if float(coefficients[icontr][iprim]) != 0.0:
+                            if iprim not in used_primitives:
+                                used_primitives.append(iprim)
+                            continue
+
+                # Form submatrices
+                reduced_exponents = [exponents[p] for p in used_primitives]
+                reduced_coefficients = [[coefficients[b][p] for p in used_primitives] for b in block]
+                redsh = sh.copy()
+                redsh['exponents'] = reduced_exponents
+                redsh['coefficients'] = reduced_coefficients
+                new_shells.append(redsh)
+
+        # Replace the shells in the basis
+        eldata['electron_shells'] = new_shells
+
+    return basis

basis_set_exchange/readers/read.py

@@ -7,6 +7,7 @@
 from ..skel import create_skel
 from ..validator import validate_data
 from ..compose import _whole_basis_types
+from ..manip import split_blocked_contractions
 from .turbomole import read_turbomole
 from .g94 import read_g94
 from .nwchem import read_nwchem
@@ -109,6 +110,9 @@ def read_formatted_basis_str(basis_str, basis_fmt, validate=False, as_component=
     if validate:
         validate_data(bs_type, data)
 
+    # Split any blocked contractions
+    data = split_blocked_contractions(data, False)
+
     return data