Monte Carlo for the electronic structure in molecules

New directions in the rapidly developing quantum Monte Carlo (QMC) electronic structure method are explored for the treatment of chemically interesting systems. Electronic correlation, a difficult problem in standard ab initio methods, is accounted for in a simple manner in QMC. Methods developed here are aimed at expanding the capabilities and applicability of QMC. Two general problems are addressed: extending QMC calculations to heavy-atom systems, and obtaining nuclear derivative and potential energy surface information. For heavy-atom systems, two methods have been developed. The first method uses ab initio effective core potentials to eliminate the core electrons. The second method retains all electrons explicitly, but achieves a separation of time scales between core and valence to speed computation. These methods are applied to atomic and molecular systems containing Li, Na, Mg, C, Si, Ge, and Fe, and results are compared to other work available. Derivative and PES information is treated in three different ways: by the Hellmann-Feynman theorem, by analytic gradient techniques, and by correlated sampling of finite differences. Each of these approaches is implemented and applied to small diatomic systems