Computational Chemistry: Reviews of Current Trends

The following sections are included:

• PREFACE

• CONTENTS

The idea of describing a many-electron system using only its electron density, i.e. without constructing its wavefunction, was initiated in the works of Thomas and Fermi. Hohenberg-Kohn theorems of modern density functional theory transformed this idea into an exact theory. The Kohn-Sham formalism, widely used in computer simulations of polyatomic systems today, is based on these theorems but is not orbital-free. It reintroduces orbitals to minimize errors in approximating the total energy. The present review concerns an alternative formalism based also on Hohenberg-Kohn theorems, in which orthogonal orbitals are used not for the whole system but only for subsystems [Cortona, Phys. Rev. B, 44 (1991) 8454]. These orbitals are derived from Kohn-Sham-like one-electron equations, called here Kohn-Sham Equations with Constrained Electron Density (KSCED), in which all terms representing the interactions between the subsystems are expressed as universal functionals of electron density. This formulation provides the formal basis for the orbital-free embedding in first-principles based multi-level simulations of complex systems, in which the orbital-level is retained for a selected subsystem, whereas its environment is described at the orbital-free level [Wesolowski and Warshel, J. Phys. Chem., 97 (1993) 8050]. The formal aspects, development and testing of relevant approximate density functionals, and the possible use of the orbital-free embedding in multi-level modelling are covered in detail in this review. Examples of applications, especially those concerning the electronic structure of embedded systems in the condensed phase are provided.

Molecules containing transition metal centers provide a complexity that leads to many phenomena of import to chemistry, physics and biology. In particular the metal centers may vary their charge and spin states leading to metal centers that may be viewed classically as closed shell unpolarized states or open-shell spin polarized ions. A large class of molecules containing collections of spin-polarized ions are often referred to as molecular magnets. Even within this somewhat specialized set of molecules, the name molecular magnet is not specific enough as there are two extremes in these systems that behave very differently when exposed to magnetic fields. In this paper we start by discussing the two idealized cases. One idealized class relates to molecules containing metal centers for which the spin-orbit interaction is quite small. The low lying excitations in this class of molecules may be studied from Heisenberg Hamiltonians. Another idealized case corresponds to molecules containing strongly coupled metal centers and non negligible spin-orbit interactions. For this case it is the spin-orbit interaction that is responsible for the lowest energy excitations in the molecule. This class of molecules has received significant experimental attention lately because it exhibits the phenomenon of resonant tunneling of magnetization which is fundamentally interesting and potentially technologically useful. We discuss how density-functional theory may be used to study this class of molecules and provide examples to current systems of interest. The examples show the magnetism in these molecules can be influenced by applied magnetic fields, addition of extra electrons and changes in hydrogen bonding. The discussion includes numerical details in addition to the underlying theoretical framework. While most of this chapter is devoted to our work on this class of anisotropic molecular magnets some discussion of the exchange-coupled idealization is included to contrast the two types of behavior.

Besides its valuable applications in surface science and to problems of chemisorption and catalysis, vibrational electron energy loss spectroscopy (EELS) is becoming a useful tool in giving information on excited states of molecules and on the electronic structure of the respective negative ions and their unimolecular decomposition. Need for a deeper chemical insight has been also recognized for a variety of electron-molecule collisional problems that have been traditionally a domain of physics. Theoretical basis for EELS is less developed than for other spectroscopies used in chemistry such as infrared, UV/VIS, or NMR. It is a feeling of the authors that the achievements of the electronic structure theory of molecules can be more exploited in the electron scattering theory. The aim of this paper is to present our attempt to develop a simple but rigorously ab initio computational method that can be applied to vibrationally inelastic electron scattering by polyatomic molecules. Some general problems of ab initio calculations on vibrational inelastic differential cross sections are discussed.

Some aspects of gas-phase reactions are discussed in a broader scientific context. Special attention is paid to reactions of free atoms and radicals, which play an important role in the modeling of the complex processes occurring in the polluted atmosphere and in combustion chemistry. Hydrogen atom abstraction and recombination reactions is the most abundant category of radical reactions of particular interest from both the theoretical and practical points of view. The computational models of the reaction rate theory most frequently used in the theoretical analysis of gas-phase reaction kinetics are briefly discussed. Selected reactions of reactive components of the polluted atmosphere, such O₂, NO_x, OH, NH₂, alkyl radicals, and halogen atoms, are discussed in detail. Kinetic parameters of the reactions under investigation are shown in an analytical form convenient for kinetic modeling studies. The expressions presented allow a successful description of the kinetics of the reaction systems in a wide temperature range.

One-bond X-H (¹J_X-H) and H-Y (^1hJ_H-Y) and two-bond X-Y (^2hJ_X-Y) spin-spin coupling constants across X-H-Y hydrogen bonds have been calculated for a variety of hydrogen-bonded complexes formed from the second period elements ¹³C, ¹⁵N, ¹⁷0, and ¹⁹F. These calculations, carried out using the ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method, have led to generalizations concerning the signs of the one- and two-bond reduced coupling constants (¹K_X-H, ^lhK_H-Y, and ^2hK_X-Y). These generalizations make possible the prediction of the signs of the corresponding spin-spin coupling constants (J) measured experimentally, taking into account the magnetogyric ratios of the pair of coupled atoms. Insight into the signs of the reduced coupling constants can be gained through the newly-proposed Nuclear Magnetic Resonance Triplet Wavefunction Model (NMRTWM).

The first step in biological activity of cisplatin, one of the most efficient anticancer drugs, involves the hydration process. This chapter reviews the results of theoretical studies concerning the activation of cisplatin and its analogs. The complexity of the studied phenomena requires application of different theoretical tools. The energetic description of the extended hydration scheme was performed for the square-planar Pt(II) and Pd(II) complexes. High-accuracy methodology was adopted for the predictions of their heats of reactions. Molecular mechanism of the ligand replacement by water molecules was investigated using the supermolecular approach. In this model the reactant metal complex was examined together with an additional water molecule. Based on such approach, not only thermodynamical but also kinetical parameters can be determined. The CCSD(T) computational level was used for the determination of the reaction energy profiles. Despite the high accuracy, it was found that the obtained results do not sufficiently match the experimental data. This causes the necessity of the improvement of the gas-phase calculations using the implicit solvent in the polarizable continuum model. The COSMO method was applied on the B3LYPreoptimized geometries and was augmented by the CCSD(T) single point calculations. These final results are in excellent accord with both the experimental data as well as other calculated values.

The following sections are included:

• INDEX

• CONTENT INDEX