Asymptotic method of building restricted open-shell wavefunctions satisfying the generalized Brillouinʹs theorem

Amended unrestricted Hartree-Fock (UHF) equations which determine an optimum set of molecular orbitals (MOs) for high-spin open-shell molecules have been obtained. The Slater determinant built from the MOs is an eigenfunction of S2 and corresponds to a minimum of energy which is equivalent to that obtained by the Roothan method. Such a set satisfies the correct variational conditions unlike other spin-restricted UHF methods. An open-shell calculation based on the MOs has a computational cost similar to closed-shell calculations.