Approximations of long-range interactions in fragment-based quantum chemical approaches

Abstract

Quantum chemical calculations of very large systems still pose major challenges due to the formidable scaling behavior of standard methods with system size. Here, we will describe how the concept of separating short- and long-range interactions can be used to make such calculations possible nonetheless at least in an approximate way. In mixed quantum mechanical/molecular mechanical (QM/MM) and fragment-based quantum chemical methods, the local surroundings are considered explicitly whereas other parts further away are neglected or included with a lower level of theory, e.g. as interactions with point charges. Different methods to combine these two descriptions, so-called embedding schemes, are outlined. Additionally, the border region problem, how subsystems describable by quantum mechanics can be generated by cleaving and saturating bonds connecting atoms located in the different regions, and proposed solutions are discussed. Finally, with the fragment-based adjustable density matrix assembler (ADMA) method as example, the capacities but also some limitations of the presented approaches will be presented using different test systems.