Bond length alternation in cyclic polyenes. II. Unrestricted hartree–fock method

Abstract

AbstractThe problem of bond length alternation in cyclic polyene models as described by the Pariser–Parr–Pople π‐electron Hamiltonian, together with an empirical quasi harmonic σ‐core potential is investigated using the unrestricted Hartree–Fock wave function employing different spatial orbitals for different spins. It is shown that in contrast to the restricted Hartree–Fock method, which favors bond alternation in large cyclic polyenes, the unrestricted Hartree–Fock method stabilizes the symmetric structures with equidistant internuclear separation. An assessment of the amount of correlation error recovered by the unrestricted Hartree–Fock procedure is examined and the qualitatively different behavior of the cyclic polyene models when described by restricted and unrestricted Hartree–Fock wave functions is discussed from this viewpoint.