<i>A</i> <i>b</i> <i>i</i> <i>n</i> <i>i</i> <i>t</i> <i>i</i> <i>o</i> Hartree–Fock crystal orbital studies. Energy bands in polyene reconsidered

Abstract

A b initio ’’exact exchange’’ LCAO Hartee–Fock (HF) energy bands are given for a polyene model with equal bond lengths. In addition to the symmetric wavefunction a broken-symmetry solution of charge density wave (CDW) type with lower total energy occurs. This instability of the metallic-type wavefunction with respect to the charge-density wave was overlooked by André and Leroy in their ab initio HF treatment of polyene. We wish to discuss the nature of the CDW and the screening effect of the σ electrons, which tend to diminish the amplitude of the CDW of the π electrons. Comparison with a model with alternant bond lengths shows the latter to be energetically more stable.