Evaluation of the OPLS-AA Force Field for the Study of Structural and Energetic Aspects of Molecular Organic Crystals

Motivated
by the need for reliable experimental data for the assessment
of theoretical predictions, this work proposes a data set of enthalpies
of sublimation determined for specific crystalline structures, for
the validation of molecular force fields (FF). The selected data were
used to explore the ability of the OPLS-AA parametrization to investigate
the properties of solid materials in molecular dynamics simulations.
Furthermore, several approaches to improve this parametrization were
also considered. These modifications consisted in replacing the original
FF atomic point charges (APC), by values calculated using quantum
chemical methods, and by the implementation of a polarizable FF. The
obtained results indicated that, in general, the best agreement between
theoretical and experimental data is found when the OPLS-AA force
field is used with the original APC or when these are replaced by
ChelpG charges, computed at the MP2/aug-cc-pVDZ level of theory, for
isolated molecules in the gaseous phase. If a good description of
the energetic relations between the polymorphs of a compound is required
then either the use of polarizable FF or the use of charges determined
taking into account the vicinity of the molecules in the crystal (combining
the ChelpG and MP2/cc-pVDZ methods) is recommended. Finally, it was
concluded that density functional theory methods, like B3LYP or B3PW91,
are not advisable for the evaluation of APC of organic compounds for
molecular dynamic simulations. Instead, the MP2 method should be considered.