CALCULATION OF MOLECULAR ELECTRONIC SPECTRA BY PROJECTED-UNRESTRICTED HARTREE-FOCK THEORY

Abstract

Unrestricted Hartree-Fock theory is developed for use in calculating electronic spectra of symmetric molecules. Excited electronic states are represented by single determinant wavefunctions, whose unrestricted nature allows for orbital rearrangement relative to the self-consistent ground state. The self-consistent calculation is followed by spin projection in order to obtain appropriate spin eigenstates. Modified procedures for portions of the spin projection calculation had to be developed because our construction of the excited state wavefunctions produces degeneracies among the natural orbitals. Illustrative calculations are provided for several pi-electron systems in the PPP approximation, and for several small molecules in the INDO approximation. The results generally compare favorably with standard configuration-interaction treatments.