Why, then, is the slowest convergence observed for the s = |m s | = 2 state? The answer is offered by the CI expansion in CSF basis. The Fe 4 S 4 cluster behaves to the leading terms as spin-system, with exchange-interactions representing the main form of spin interactions across the sites. CSFs with singly occupied orbitals represent the leading configurations of such a spin-system, and their number is promptly given by the van Vleck-Sherman formula: 99