Improved intermolecular force field for crystalline hydrocarbons containing four- or three-coordinated carbon

A new intermolecular force field of the (exp-6-1) type was derived from observed hydrocarbon crystal structures selected from a comprehensive set of 134 molecules with 5-16 carbon atoms, and eight observed heats of sublimation. Carbon atoms with four bonds, C(4), and those with three bonds, C(3), were assigned separate parameters. Wavefunctions were calculated for each molecule and the molecular electric potential was modelled with net atomic charges. Methylene bisector or ring center site charges were added as necessary to fit the molecular electric potential. Each crystal structure was relaxed by energy minimization; for 101 of the structures the force field predicted changes in unit cell edge lengths less than 3%. The remaining 33 crystal structures, while otherwise meeting data quality criteria, showed predicted unit cell edge length changes greater than 3% and were not included in the training set. (C) 1999 Elsevier Science B.V. All rights reserved.