The multiconfiguration half-projected Hartree–Fock (MCHPHF) method and its application to low-lying singlet excited states of ketene, allene and benzene

Abstract

Numerous applications of the half-projected Hartree–Fock (HPHF) method have demonstrated that this approach is appropriate for determining low-lying singlet excited states of molecules. The computational effort is comparable to that with a standard unrestricted Hartree–Fock (UHF) calculation. To circumvent the problems of molecules with degenerate or quasi-degenerate excited states, the HPHF method has been formulated by introducing a multiconfigurational ansatz for the linear combination of projected two-determinantal wave functions. Calculation of the first excited states of π-electron systems results in excitation energies that are close to experimental data.