# FCI implementation

Could you suggest, please, a manual/textbook/tutorial for implementing FCI (full configuration interaction) method on computers. I am interesting in algorithms mainly. I would like to write my own code, so I want to study how one can provide convenient indexing for configurations and what tricks are usually used to decrease computation time.

## 3 Answers

MOLPRO has a full CI module (by Peter Knowles), but it also costs quite a bit of money. You can also do full CI in GAMESS (I think Ruedenberg's implementation), and perhaps any MCSCF/CASSCF program that is efficient enough to include all orbitals in the active space.

I regret to inform you: you may be in for a lot of work if try to write your own code. :-( This is an area of very deep, highly mathematical and numerical concepts.

So as far as a manual/textbook: You may want to try Molecular-Electronic Structure Theory, especially Chapter 11 and 12. There is just a crazy amount of work that you will have to do prior to CI, including the coding of the various four-center integrals for electron repulsion, kinetic energy, and overlap (earlier chapters).

• Molpro is not nearly as expensive as truly commercial packages like Gaussian. As one can see on molpro.net/info/…, a single workstation license costs $350/$600 for the serial/parallel version. That's about the same price as Microsoft Office was the last time I looked (close to 10 years ago). – Jeff Oct 23 '13 at 14:52

I happen to have a simple FCI code with detailed documentation of implementation. You can find it at http://scholar.princeton.edu/boxiao/software/fci-201302