Intermolecular electrostatics via molecular fragmentation: an application to crystal structure prediction

Molecular fragmentation is an attractive approach to the simulation of large molecules, in which calculations are carried out on small segments of the molecule, achieving linear scaling but reduced accuracy. Its application to crystal structure prediction (CSP) is challenged by high accuracy requirements. In this study, the applicability of a fragmentation scheme is tested for distributed multipoles, which are used in CSP to model intermolecular electrostatic interactions. Four test systems are investigated: a molecular salt, the highly conjugated molecule retinal, a model pharmaceutical molecule and the nonlinear molecule nitrotriacetanilide. It is demonstrated that fragment-based electrostatics reproduce, to an acceptable degree, a set of crystal structures generated using whole-molecule electrostatics. Inclusion of the molecular environment of each fragment out to four bonds separation is found to provide a sufficiently accurate set of distributed multipoles for the purposes of CSP.