SISO Code Cleanup (#220)

* Removing the un-necessary mutation of mc object for L-PDFT

* Add SF-TDDFT nonadiabatic couplings and batch caching (#210)

* feat: add spin-flip TDDFT NAC and batch caching

Add spin-flip nonadiabatic coupling support, gradient/NAC cache helpers,
batch APIs, and regression tests. Resolve Ruff and Flake8 findings.

* fix: rely on the PySCF tools namespace package

* refactor: scope gradient/NAC caching to NAC package

Move the batch and cache utilities under pyscf.nac, replace the module-level
cache singleton with an explicitly scoped context manager, and add focused
unit tests for cache reuse, geometry keys, isolation, and cleanup.

* fix: preserve NAC API and validate cache correctness

---------

Co-authored-by: 井与 <2100011839@stu.pku.edu.cn>

Author: JangidBhavnesh

pyscf/siso/siso.py

@@ -583,9 +583,6 @@ def sanity_checks(self):
         if self.amf == self.mmf:
             raise ValueError("Exactly one of amf and mmf must be enabled")
 
-        if isinstance(self.mc, mcpdft.MultiStateMCPDFTSolver):
-            self.sort_eigenstates_forlpdft()
-
         expected_nroots = sum(state[0] for state in self.modelspace)
         solvers = getattr(self.mc.fcisolver, 'fcisolvers', None)
         if solvers is not None:
@@ -638,15 +635,6 @@ def sanity_checks(self):
         self._state_s2 = ss
         return self
 
-    def sort_eigenstates_forlpdft(self):
-        '''
-        Sorting the L-PDFT ci-vecs and energies accoarding to spin-states.
-        '''
-        ss = self.mc.fcisolver.states_spin_square(self.mc.ci, self.mc.ncas, self.mc.nelecas)[0]
-        idx = np.argsort(ss)
-        self.mc.ci = [self.mc.ci[i] for i in idx]
-        self.mc.e_states = [self.mc.e_states[i] for i in idx]
-
     def dump_flags(self):
         log = logger.Logger(self.mc.stdout, self.mc.verbose)
         log.note(" ")