Semiempirical and Molecular Mechanics Treatment of Noncovalent Interactions

The importance of non-bonding interactions between and also within molecular systems has come into the focus of many researchers over the last two decades. The important interactions are first order exchange-repulsion, (non-polarized) electrostatic and second order polarization/induction (up to charge-transfer) as well as dispersion interactions. This chapter gives a short review of theoretical basics of non-covalent interactions. It presents an overview of features of non-polar interactions and polar interactions. The chapter discusses the general aspects of non-covalent interactions for molecular modelling methods and semi-empirical quantum mechanical methods. The Grimme ansatz is used for semi-empirical quantum mechanical and density functional tight binding (DFTB) methods as well as periodic DFT calculations in solid-state physics and surface science. The chapter focuses on the treatment of the most complicated interactions, namely dispersion and hydrogen-bond interactions. Two semi-empirical quantum mechanical methods make use of the first-generation hydrogen bond corrections, i.e., PM6-DH and AM1-FS1. Keywords: density functional tight binding method; dispersion interactions; hydrogen-bond interactions; molecular modelling methods; non-covalent interactions; polar interactions; semi-empirical quantum mechanical methods