P. Cassamchenai et Gs. Chandler, SPIN-UNRESTRICTED CALCULATIONS IN QUANTUM-CHEMISTRY, International journal of quantum chemistry, 46(5), 1993, pp. 593-607
The unrestricted complete active space self-consistent field (UCASSCF)
function is defined, and a proof that a UCASSCF eigenfunction of the
spin operator S2 is a CASSCF function is given. The spin-contamination
for an unrestricted Hartree-Fock (UHF) function is evaluated by using
Araki angle operators, and the UHF function is then projected on the
restricted open-shell Hartree-Fock (ROHF) space. The present analysis
has deep consequences since it implies that the only non-spin-contamin
ated UHF functions are the ROHF functions. This is illustrated in a ca
lculation of the spin density of He2+.