Date of Award
Doctor of Philosophy
Mark S. Gordon
For many problems, especially large ones such as discrete solute/solvent interactions, traditional quantum chemical methods become unsuitable because of the computational time required. In order to study such systems, hybrid quantum mechanics/molecular mechanics techniques have been developed, such as the Effective Fragment Potential, which is described in detail. One of the interactions that must be accounted for using this method is charge penetration between distributed multipolar expansions, and this is derived and tested in Chapter 2.;A powerful experimental spectroscopy, nuclear magnetic resonance (NMR), measures the so-called chemical shifts of nuclei in the presence of a magnetic field. It is explained in Chapter 3 how the physical property of chemical shifts are a second derivative of the energy with respect to the external magnetic field and the nuclear magnetic moments, and this derivative is evaluated in detail. The resulting integrals are further evaluated using the McMurchie-Davidson method, and coded into the quantum chemistry package GAMESS using the modified algorithm described. The chemical shifts are also shown to depend on the first derivative of the density matrix, whose evaluation is seen to require antisymmetric perturbation theory for nonorthogonal, perturbation-dependent basis sets. This formalism is also derived in Chapter 3. Finally, it is explained how our ultimate goal is to predict chemical shifts in solution, so several possible approaches are given on how to integrate the EFP method with the GIAO formalism.;A conventional application of the EFP method is given in Chapter 4, which studies the solvation of formic and acetic acids. Although the preliminary results of this study are promising, it is noted that further progress in this area awaits an appropriate Monte Carlo or molecular mechanics code because of the currently inadequate method for sampling configuration space.;Chapter 5 describes multi-reference versions of the so-called G2 and G3 methods and reports their performance in detail.;Finally, Chapter 6 discusses the electronic structure of the complex organometallic molecule titanocene. The history of the study of this species is discussed, and the results of many high level calculations such as DFT and MP2 with a triple-zeta basis set are evaluated.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Mark Alan Freitag
Freitag, Mark Alan, "From first principles: the application of quantum mechanics to complex molecules and solvated systems " (2002). Retrospective Theses and Dissertations. 370.