Classical molecular dynamics using predefined potentials, force fields, either based on empirical data or on independent electronic structure calculations, is well established as a powerful tool serving to investigate many-body condensed matter systems, including biomolecular assemblies. The broadness, diversity, and level of sophistication of this technique are documented in several books as well as review articles, conference proceedings, lecture notes, and special issues [25, 120, 136, 272, 398, 468, 577, 726, 1189, 1449, 1504, 1538, 1539]. At the very heart of any molecular dynamics scheme is the question of how to describe – that is in practice how to approximate – the interatomic interactions. The traditional route followed in molecular dynamics is to determine these potentials in advance. Typically, the full interaction is broken up into two-body and many-body contributions, long-range and short-range terms, electrostatic and non-electrostatic interactions, etc., which have to be represented by suitable functional forms, see Refs. [550, 1405] for detailed accounts. After decades of intense research, very elaborate interaction models, including the nontrivial aspect of representing these potentials analytically, were devised [550, 1280, 1380, 1405, 1539].

Despite their overwhelming success – which will, however, not be praised in this book – the need to devise a fixed predefined potential implies serious drawbacks [1123, 1209]. Among the most significant are systems in which (i) many different atom or molecule types give rise to a myriad of different interatomic interactions that have to be parameterized and/or (ii) the electronic structure and thus the chemical bonding pattern changes qualitatively during the course of the simulation. Such systems are termed here “chemically complex”.