Comparison Between the Performances of the Spin-Projected Hartree-Fock, Generalized Valence-Bond, and Spin-Coupled Approaches

The total energies from spin-projected Hartree-Fock (spin-PHF), generalized valence-bond with perfect-pairing and strong orthogonality restrictions (GVB-PP-SO), and fully-variational spin-coupled (SC) calculations are compared for a series of unsaturated alternant hydrocarbons, ranging from trans buta-1,3-diene to naphthalene. The extent to which the spin-PHF and GVB-PP-SO wavefunctions for these molecules can approximate the more general SC wavefunction can be understood by analyzing the weights of the different modes of spin coupling in the full SC treatment. The spin-PHF wavefunction is found to display a Surprising ability to capture more of the pi-space correlation effects in aromatic and antiaromatic systems than its GVB-PP-SO counterpart, even in the case of naphthalene, a molecule with 10 pi electrons. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 2447-2455, 2009