Calculation of molecular electronic spectra by projected‐unrestricted Hartree–Fock theory

Abstract

AbstractThis paper is concerned with a new application of projected‐unrestricted Hartree–Fock theory, namely, the calculation of electronic spectra for symmetric molecules. The excited electronic state is represented by a single determinant whose unrestricted nature allows for orbital rearrangement relative to the self‐consistent ground state. The self‐consistent calculation must be followed by spin projection to obtain appropriate spin eigenstates. It was necessary to develop modified procedures for portions of the spin projection calculation because our method of constructing the wave functions produces degeneracies among the natural orbitals. Illustrative calculations using the all‐valence‐electron INDO approximations produced results which compared favorably with configuration‐interaction treatments. The method described here should be most useful, however, in conjunction with ab initio calculations using flexible basis sets.