DL_MULTI—A molecular dynamics program to use distributed multipole electrostatic models to simulate the dynamics of organic crystals

Abstract

DL_MULTI has been developed to extend the Molecular Dynamics simulation program DL_POLY to model rigid molecules whose intermolecular interactions include a distributed multipole model for the electrostatic interactions. The adaptations use anisotropic atom–atom potentials, corresponding to atomic multipoles up to hexadecapole. The lattice sums of these multipoles are evaluated using the Ewald method, using a technique utilizing Stone's S functions which describe the multipoles in a molecule fixed reference frame. An algorithm for determining suitable cutoffs is described and errors in the direct space part of the Ewald summation discussed. Thus DL_MULTI provides a general purpose MD program for studying polar rigid organic molecules in their liquid and crystalline states with a realistic intermolecular potential suitable for studying polymorphism. Example applications to uracil and 5-azauracil show that, with the new summation method, a realistic electrostatic model can be used without excessive computer time.

Keywords:

• intermolecular forces
• organic crystal structure prediction
• polymorphism

Acknowledgements

Stephen Spence, Graeme Day, Aileen Gray and Sarah (Sally) L. Price are thanked for their assistance in developing and testing DL_MULTI whilst funded by an EPSRC ROPA grant. Dr. W. Smith also gave valuable advice.