Ab initio energies of nonconducting crystals by systematic fragmentation

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 SiO₂ 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

Drama in dynamics: boom, splash, and speed

The full nature of theoretical chemistry and physics cannot be captured by static calculations alone. The underlying goal of this thesis is the utilization of dynamics methods, appropriate to the size and type of chemical system under consideration, as well as the type of desired data obtained from the calculation. A small, potentially high-energy molecule, FN5, was studied with high level, on-the-fly ab initio (AI) methods in order study isomerization and decomposition pathways, and ultimately predict the existence (lifetime) of the species. Experimentalists confirmed these calculations. In order to study solvation processes, large numbers of molecules, as well as dynamics methods, are important. The Effective Fragment Potential (EFP) method for solvation, a method based on quantum mechanics calculations, was parallelized to facilitate these calculations within the quantum chemistry program GAMESS. The parallel algorithm employs both atom decomposition of data, as well as non-blocking communication. Speedup and scalability of the code was achieved. The EFP method has been shown to provide excellent results for small water clusters in previous calculations. In order to test the bulk behavior of the EFP method, EFP molecular dynamics simulations were performed. The resulting radial distribution functions for water are in good agreement with experimental data. Finally, one of the most fundamental aspects of a chemical reaction was investigated: the molecular potential energy surface (PES). This involved the interface of the Grow and GAMESS programs. Grow builds a PES as an interpolation of AI data, and thus requires AI calculations of energy and derivatives from GAMESS. Classical or quantum dynamics can be performed on the resulting surface. The interface also includes the capability to build multi-reference PESs; these types of calculations are applicable to a wide array of problems, including photochemistry and photobiology

Growing multiconfigurational potential energy surfaces with applications to X+H₂ (X=C,N,O) reactions

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 + H2 reactions.This work was supported by a National Science Foundation Foreign Travel Grant and a Fulbright Senior Scholar Award to one of the authors M.S.G. and by a grant from the Air Force Office of Scientific Research to one of the authors M.S.G. . Another author H.M.N. was supported by a Department of Energy Computational Science Graduate Fellowship

Gradients of the Polarization Energy in the Effective Fragment Potential Method

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

Growing Multiconfigurational Potential Energy Surfaces with Applications to X+H2 (X=C,N,O) Reactions

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

Development of High Performance Scientific Components for Interoperability of Computing Packages

Three major high performance quantum chemistry computational packages, NWChem, GAMESS and MPQC have been developed by different research efforts following different design patterns. The goal is to achieve interoperability among these packages by overcoming the challenges caused by the different communication patterns and software design of each of these packages. Developing a chemistry algorithm is a time consuming process; integration of large quantum chemistry packages will allow resource sharing and thus avoid reinvention of the wheel. Creating connections between these incompatible packages is the major motivation of our work. We achieve this interoperability by bringing the benefits of Component Based Software Engineering through a plug-and-play component framework called Common Component Architecture (CCA). In this paper, we present a strategy and process used for interfacing two widely used and important computational chemistry methodologies: Quantum Mechanics and Molecular Mechanics. This paper also demonstrates the performance evaluation of these CCA compliant components to show the feasibility of the proposed approach and finally discusses the current research issues

The Effective Fragment Potential: Small Clusters and Radial Distribution Functions

The effective fragment potential (EFP) method for treating solventeffects 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