An experimental chemist's guide to ab initio quantum chemistry

Abstract

This article is not intended to provide a cutting edge, state-of-the-art review of ab initio quantum chemistry. Nor does it offer a shopping list of estimates for the accuracies of its various approaches. Unfortunately, quantum chemistry is not mature or reliable enough to make such an evaluation generally possible. Rather, this article introduces the essential concepts of quantum chemistry and the computationalfeatures that differ among commonly used methods. It is intended as a guide for those who are not conversant with the jargon of ab initio quantum chemistry but who are interested in making use of these tools. In sections I-IV, readers are provided overviews of (i) the objectives and terminology of the field, (ii) the reasons underlying the often disappointing accuracy of present methods, (iii) and the meaning of orbitals, configurations, and electron correlation. The content of sections V and VI is intended to serve as reference material in which the computational tools of ab initio quantum chemistry are overviewed. In these sections, the Hartree-Fock (HF), configuration interaction (CI), multiconfigurational self-consistent field (MCSCF), Maller-Plesset perturbation theory (MPPT), coupled-cluster (CC), and density functional methods such as X, are introduced. The strengths and weaknesses of these methods as well as the computational steps involved in their implementation are briefly discussed. 1. What Does ab Initio Quantum Chemistry Try To Do? The trends in chemical and physical properties of the elements described beautifully in the periodic table and the ability of early spectroscopists to fit atomic line spectra by simple mathematical formulas and to interpret atomic electronic states in terms of empirical quantum numbers provide compelling evidence that some relatively simple framework must exist for understanding the electronic structures of all atoms. The great predictive power of the concept of atomic valence further suggests that molecular electronic structure should be understandable in terms of those of the constituent atoms. This point of view lies at the heart of modern chemistry. Much of ab initio quantum chemistry attempts to make more quantitative these aspects of chemists’ views of the periodic table and of atomic valence and structure. By starting from ”first principles” and treating atomic and molecular states as solutions of the Schriidinger equation, quantum chemistry seeks to determine what underlies the empirical quantum numbers, screening, quantum defects, the aufbau principle, and the concept of valence used by spectroscopists and chemists, in some cases, even prior to the advent of quantum mechanics. The discipline of computational ab initio quantum chemistry is aimed at determining the electronic energies and wave functions of atoms, molecules, radicals, ions, solids, and all other chemical species. The phrase ab initio implies that one attempts to solve the Schriidinger equation from first principles, treating the molecule as a collection of positive nuclei and negative electrons moving under the influence of Coulombic potentials and not using any prior knowledge about this species’ chemical behavior.