Computational Chemistry: Reviews of Current Trends

The following sections are included:

• PREFACE

• CONTENTS

Electron propagator theory provides a framework for the systematic inclusion of electron correlation in a one–electron picture of molecular electronic structure. Propagator calculations produce Dyson orbitals and correlated electron binding energies without determining wave-functions and energies of individual states. Several approximate propagators are derived and are shown to be accurate and efficient tools for the computation of vertical and adiabatic electron binding energies. The association of Dyson orbitals to electron binding energies facilitates interpretation of electronic structure in terms of one–electron concepts.

Theoretical aspects of the SAC/SAC-CI method for calculating ground, excited, ionized, and electron attached states of molecules are summarized. The SAC-CI SD (singles and doubles) method for the ground and excited states of singlet to septet spin multiplicities and the SAC-CI general-R method for two-to-many electron excitation processes are described and their accuracies are explained based on the computed results. Some recent topics of interest in the applications such as the spectra of metal complexes, the collision induced absorption spectra of CsXe, and the excitation spectra of porphyrins are reviewed. The SAC/SAC-CI method is shown to be simple enough to be useful and accurate enough to be useful.

We discuss Monte Carlo methods applicable to the electronic structure of atoms and molecules. We summarize accomplishments of the past 15 years and indicate challenges that remain. We discuss both the fixed-node and released-node methods and focus on first-row atoms and selected homonuclear diatomics in illustrating capabilities and achievements.

Results are presented from our recent ab initio quantum-mechanical calculations (conventional Hartree-Fock (HF) and post-Hartree-Fock (MP2) methods, density functional theory with the combined Becke3-LYP exchange-correlation energy functional (DFT(B3LYP) method)) applied to predict molecular parameters (geometries, rotational constants, dipole moments) and IR spectra (harmonic wavenumbers, absolute intensities) of the DNA bases, their several derivatives, and model systems. The results of the calculations are compared with the available experimental data and with the infrared spectra of the species isolated in low-temperature inert-gas matrices. Molecular parameters predicted by the DFT(B3LYP) method agree well with the experimental data and with the parameters predicted by the MP2 calculations. The IR spectra of the bases calculated by the DFT(B3LYP) method agree much better with the recorded experimental spectra than do the spectra predicted at the HF level and also are better than those predicted at the MP2 level. The results of calculations are also used to interpret the tautomeric stabilities of the bases.

To understand specific interactions between biological molecules, systematic theoretical studies are of importance. It is impracticable, however, to apply ab initio molecular orbital methods to these objects because of the size and the complexity. In order to apply modern computational chemistry to large biological systems, it is necessary to develop a set of analytical potentials based on theab initio MO method and to apply molecular dynamics simulations using those potentials. In this article, we focus on the derivation of such a set of new potentials and its assessment concerning the hydrogen bonding and ion-pair interactions. Biological molecules vary their conformations according to circumstances. It is demonstrated that the new set of ab initio potentials is sensitive enough to describe the conformational changes depending on the interactions between surrounding molecules.

Teaching quantum chemistry has become an important issue, and the demand for textbooks and practical exercises at different levels of skill and background has risen substantially. At the same time, education is being revolutionized by new technologies available in electronic media. Information networks are changing the way educational programs are developed and disseminated. In this work we present the practical exercises for the ab initio quantum chemistry course at the Department of Chemistry of ETH, Zürich. The course gives an introduction to the methods of ab initio quantum chemistry with the goal of enabling students to perform their own ab initio studies. The lectures are accompanied by practical exercises in which the student solves small problems providing insight into the various theoretical methods, the techniques of their application, and the interpretation of results. The practical exercises, which cover a broad range of topics from different areas of physical chemistry, are discussed here, as are the reasons for providing them as hypertext documents. The actual assignments can be found on the World Wide Web at http://www.scsc.ethz.ch/chem/qcii.html.

The following sections are included:

• INDEX