Pi-Electron Theory of Acetylene. II. Unrestricted Hartree—Fock Theory

Abstract

Unrestricted Hartree—Fock (UHF) theory was applied to a Pariser—Parr—Pople pi-electron Hamiltonian for acetylene. The symmetries that must be broken in order to obtain the best UHF ground state were examined as a function of the Hamiltonian parameters. Improved ground-state wavefunctions were obtained for this two-band system by projecting out the undesired symmetries. Ground-state energies and correlation functions of the symmetrized UHF states were compared to the exact diagonalization of the Hamiltonian. For the model used, electron correlation is very well represented by the symmetrized UHF states.