Direct minimization of the energy functional in LCAO-MO density matrix formalism

Abstract

A general theory is presented for the optimization of the coefficients of orbitals and configuration interaction expansion in the case of multiconfiguration wavefunctions containing all single excitations. The orbital coefficients are optimized by suitable orthogonal transformations of the atomic basis; the Cl coefficients are determined solving the usual secular problem. The energy minimization is performed directly by a gradient approach. The method works both for ground and excited states and no convergence difficulties are met. Computational examples are given for H₂O and H₂S molecules.