Selfconsistent Theory of Bond Alternation in Polyenes: Normal State, Charge-Density Waves and Spin-Density Waves

摘要

The unrestricted Hartree-Fock theory is used to investigate bond alternation in C(4N+2)H(4N+2) cyclic polyenes. The possible ground states are examined as functions of the parameters in the Pariser-Parr-Pople model hamiltonian. It is shown that charge density wave ground states do not exist for physical values of the input parameters. The normal, or molecular orbital ground state, does give rise to a negligible bond alternation at large N, in agreement with previous workers. Spin density wave ground states exist for most values of the parameters and have lower energy than the normal ground states. These states do not exhibit bond alternation when the sigma bond compression is the same as that used to obtain the normal energy. (Author)