Weak intermolecular interactions: A supermolecular approach

Abstract

Weak intermolecular interactions, which are ubiquitous in biological and materials chemistry, are fast becoming more routinely and accurately investigated owing to the increased performance of computational methods being actively developed. A vast array of pragmatic methods have been proposed using empirical, semi-empirical, density functional theory, and ab initio approaches, which all serve to widen the scope of feasible problems. Especially for the calculation of the important London dispersion interactions, significant progress has been achieved. Herein, we present a general overview on a number of illustrative strategies used to routinely investigate structures and energies of such systems. The composition and advantages/disadvantages of different benchmark sets, which have been found to be of crucial importance in assessing such a wide range of methods is discussed. Finally, a number of experience-based perspectives are provided in relation to the scaling and accuracy of the “more popular” methods used when investigating non-covalent interactions. A present trend in quantum chemistry is on cheap and reliable methods that effectively solve present-day problems in biological and materials chemistry. Quantum chemistry now confidently looks beyond small polyatomic molecules and toward large supramolecular complexes; this represents an area on the cutting edge of simulation sciences. This chapter deals with weak intermolecular (non-covalent) interactions between molecules in the gas phase. These interactions are essential for the quantitative description and understanding of complex molecular aggregates in physics (e.g., surface science), chemistry, and molecular biology. The same interactions also occur in an intramolecular fashion between atoms or groups in one molecule. One of the big advantages of the supermolecular approach described herein is that it can handle both situations on an equal footing. Just for convenience and due to space limitations, we will consider here only intermolecular cases (complexes of at least two molecules). The reader should, however, keep in mind that much of what we are saying about quantum chemical methods similarly holds for the quantum chemical simulation of protein folding. The following chapter is a pragmatic overview on “current” methods that are useful in obtaining reliable data from quantum chemical calculations, with a strong focus on methods used (and developed) primarily to study such non-covalent interactions. Weak intermolecular interactions in the solid or solution phase are almost completely neglected here, this is by no means a reflection on their importance, rather a way of restricting the scope of this chapter to a particular stream of research. A thorough description of the underlying theory of molecular interactions is presented in Volume I written by Alston Misquita. Only a succinct overview of weak intermolecular interactions is given below to “set the scene.”