First steps towards adding full configuration interaction

[jarvist]

Required for CI:-

• [ ] Transform 2e integrals from atomic basis to molecular orbitals (N^5 process is done naively).
• [ ] Matrix elements between Slater determinants. Either via Slater-Condon rules (see Szabo and Ostlund), or the full Slater-Condon-Harris rules
• [ ] Solve the new secular / matrix equation. (Nb: for any serious calculation, need to exploit sparsity + possibly locality, to avoid the N! scaling)

References:- c.f. (old) pyquante 'CI.py' - this is a pretty good reference for the singles at least http://pyquante.sourceforge.net/ Chapter 4 in Szabo and Ostlund Sherrill notes http://vergil.chemistry.gatech.edu/notes/ci.pdf