A systematic method for approximating the ab initio electronic energy of molecules from the energies of molecular fragments has been adapted to estimate the total electronic energy of crystal lattices. The fragmentation method can be employed with any ab initio electronic structure method and allows optimization of the crystal structure based on ab initio gradients. The method is demonstrated on SiO(2) polymorphs using the Hartree-Fock approximation, second order Moller-Plesset perturbation theory, and the quadratic configuration interaction method with single and double excitations and triple excitations added perturbatively .

The effective fragment potential (EFP) method is an ab initio based polarizable classical method in which the intermolecular interaction parameters are obtained from preparative ab initiocalculations on isolated molecules. The polarization energy in the EFP method is modeled with asymmetric anisotropic dipole polarizabilitytensors located at the centroids of localized bond and lone pair orbitals of the molecules. Analytic expressions for the translational and rotational gradients (forces and torques) of the EFP polarization energy have been derived and implemented. Periodic boundary conditions (the minimum image convention) and switching functions have also been implemented for the polarization energy, as well as for other EFP interaction terms. With these improvements, molecular dynamics simulations can be performed with the EFP method for various chemical systems. The effective fragment potential ͑EFP͒ method is an ab initio based polarizable classical method in which the intermolecular interaction parameters are obtained from preparative ab initio calculations on isolated molecules. The polarization energy in the EFP method is modeled with asymmetric anisotropic dipole polarizability tensors located at the centroids of localized bond and lone pair orbitals of the molecules. Analytic expressions for the translational and rotational gradients ͑forces and torques͒ of the EFP polarization energy have been derived and implemented. Periodic boundary conditions ͑the minimum image convention͒ and switching functions have also been implemented for the polarization energy, as well as for other EFP interaction terms. With these improvements, molecular dynamics simulations can be performed with the EFP method for various chemical systems.

The effective fragment potential (EFP) method for treating solvent effects provides relative energies and structures that are in excellent agreement with the analogous fully quantum [i.e., Hartree-Fock (HF), density functional theory (DFT), and second order perturbation theory (MP2)] results for small water clusters. The ability of the method to predict bulk water properties with a comparable accuracy is assessed by performing EFP molecular dynamics simulations. The resulting radial distribution functions (RDF) suggest that as the underlying quantum method is improved from HF to DFT to MP2, the agreement with the experimental RDF also improves. The MP2-based EFP method yields a RDF that is in excellent agreement with experiment.

A previously developed method, based on a Shepard interpolation procedure to automatically construct a quantum mechanical potential energy surface (PES), is extended to the construction of multiple potential energy surfaces using multiconfigurational wave functions. These calculations are accomplished with the interface of the PES-building program, GROW, and the GAMESS suite of electronic structure programs. The efficient computation of multiconfigurational self-consistent field surfaces is illustrated with the C+H2, N+H2, and O+H2reactions. KeywordsSurface states, Hydrogen reactions, Interpolation, Potential energy surfaces, Exchange reactions Disciplines Chemistry CommentsThe following article appeared in Journal of Chemical Phsyics 124, 154104, and may be found at doi:10.1063/ 1.2185641. Growing multiconfigurational potential energy surfaces with applications to X + H 2 "X = C , N , O… reactions A previously developed method, based on a Shepard interpolation procedure to automatically construct a quantum mechanical potential energy surface ͑PES͒, is extended to the construction of multiple potential energy surfaces using multiconfigurational wave functions. These calculations are accomplished with the interface of the PES-building program, GROW, and the GAMESS suite of electronic structure programs. The efficient computation of multiconfigurational self-consistent field surfaces is illustrated with the C + H 2 , N+H 2 , and O + H 2 reactions.