Enforce incore ERIs when cluster has no excitations in one spin channel.

vayesta/solver/ccsd.py

@@ -363,5 +363,21 @@ def _debug_exact_wf(self, wf):
         self.wf = CCSD_WaveFunction(mo, t1, t2, l1=l1, l2=l2)
         self.converged = True
 
+    def get_eris(self):
+        """Get ERI object."""
+
+        # This is to compensate for possible issues with PySCF's UCCSD._make_df_eris_outcore method when one spin
+        # channel has no occupied and/or virtual orbitals.
+        try:
+            return super().get_eris()
+        except ValueError as e:
+            if any(np.array(self.cluster.nocc_active) == 0) or any(np.array(self.cluster.nvir_active) == 0):
+                self.log.info("Cluster has no occupied and/or virtual orbitals in one spin channel.")
+                with replace_attr(self.solver, incore_complete=True):
+                    return super().get_eris()
+            else:
+                # Need to ensure we raise the error that will otherwise be suppressed.
+                raise e
+
     def _debug_random_wf(self):
         raise NotImplementedError

vayesta/solver/cisd.py

@@ -89,7 +89,7 @@ def make_rdm2(self, *args, **kwargs):
         raise NotImplementedError()
 
 
-class UCISD_Solver(CISD_Solver):
+class UCISD_Solver(CISD_Solver, ccsd.UCCSD_Solver):
 
     def get_solver_class(self):
         # No DF version for UCISD