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

The unrestricted Hartree–Fock theory is used to investigate bond alternation in C4N+2H4N+2 cyclic polyenes. The possible ground states are examined as functions of the parameters in the Pariser–Parr–Pople model Hamiltonian. We show 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.