Spin‐unrestricted calculations in quantum chemistry

AbstractThe 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‐contaminated UHF functions are the ROHF functions. This is illustrated in a calculation of the spin density of He. © 1993 John Wiley & Sons, Inc.