45403-AC6
Development of Partially Spin-Restricted Geminal Model for Studies of Transition Metal Compounds
The spin purification procedure based on the geminal reference wave function was developed in the previous year of this Grant. We have realized that this procedure can be applied to the much more popular spin-unrestricted Hartree-Fock (UHF) wave function. The UHF can be transformed into corresponding orbitals, which are defined as orbitals with diagonal overlap matrix between alpha and beta spin-orbitals. Each orbital pair with non-zero overlap is then written as partially spin restricted geminal, and the spin purification procedure applies. We have coded this method into a development version of Q-Chem program, and are now in the process of computing energies and the Fermi contact spin densities of the various radicals. The computational cost of the procedure is dominated by the atomic-to-molecular orbital transformation step. Therefore, it can be applied to large systems, such as copper-oxygen containing compounds, which are known to have multiple spin-polarized bonds. The two major outstanding issues are: (i) is it practical to combine the new procedure with orbital optimization, and (ii) is it practical to apply perturbative corrections to the spin-purified UHF-based wave function. The preliminary calculations on the diatomic pi radicals suggest that orbital re-optimization may be important. We have presented this work in July of 2008 in Vancouver, Canada at the 6th Congress of the International Society for Theoretical Chemical Physics, and in September of 2008 in Sydney, Australia at the 2008 Congress of the World Association of Theoretical and Computational Chemists.
