A simplified direct minimization of SCF energy for many-shell systems

Abstract

A simplified version of the many-shell SCF theory is presented in which advantage is taken of the direct mixing between occupied and virtual orbitals and of the invariance to orthogonal mixing within each shell subspace. A vector gradient and a second derivative matrix of dimensions equal to the number of the occupied orbitals are used in a variable metric and Newton-Raphson algorithms to minimize the energy functional. Numerical tests are given for the ²II state (three shells) of CH and for the LiH molecule.