Guardrail against combine_pair misuse

Raise an exception if trying to combine two keyframes that are
responsible for the same fragment

my_pyscf/mcscf/lasscf_async/combine.py

@@ -208,6 +208,10 @@ def combine_pair (las, kf1, kf2, kf_ref=None):
     '''Combine two keyframes and relax one specific block of active-active orbital rotations
     between the fragments assigned to each with the inactive and virtual orbitals frozen.'''
     if kf_ref is None: kf_ref=kf1
+    if len (kf1.frags.intersection (kf2.frags)):
+        errstr = ("Cannot combine keyframes that are responsible for the same fragments "
+                  "({} {})").format (kf1.frags, kf2.frags)
+        raise RuntimeError (errstr)
     kf3 = orth_orb (las, [kf1, kf2], kf_ref=kf_ref)
     i, j = select_aa_block (las, kf1.frags, kf2.frags, kf3.fock1)
     kf3 = relax (las, kf3, freeze_inactive=True, unfrozen_frags=(i,j))

my_pyscf/mcscf/lasscf_async/keyframe.py

@@ -349,5 +349,24 @@ def democratic_matrix (las, mat, frags, mo_coeff):
 
     return u @ mat @ u.conj ().T
 
+# Thought I might need this; realize I don't. Might still be useful later.
+def fock_cycle (las, kf1):
+    '''For the inactive-virtual orbital rotations only, build and diagonalize the fock
+    matrix once'''
+    nao, nmo = kf1.mo_coeff.shape
+    ncore, ncas = las.ncore, las.ncas
+    nocc = ncore + ncas
+    nvirt = nmo - nocc
+    mo = np.append (kf1.mo_coeff[:,:ncore], kf1.mo_coeff[:,nocc:])
+    if not mo.shape[1]: return kf1
+    kf2 = kf1.copy ()
+    fock = las.get_hcore ()[None,:,:] + kf1.veff
+    fock = get_roothaan_fock (fock, kf1.dm1s, las._scf.get_ovlp())
+    orbsym = None # TODO: symmetry
+    fock = mo.conj ().T @ fock @ mo
+    ene, umat = las._eig (fock, 0, 0, orbsym)
+    if ncore: kf2.mo_coeff[:,:ncore] = mo @ umat[:,:ncore]
+    if nvirt: kf2.mo_coeff[:,nocc:] = mo @ umat[:,ncore:]
+    return kf2