Improved intermolecular force field for crystalline oxohydrocarbons including OH???O hydrogen bonding

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An improved intermolecular force field of the (exp-6-1) type was obtained by fitting a training set of 124 observed oxohydrocarbon crystal structures and seven observed heats of sublimation. All of these structures, when energy minimized, showed cell edge length shifts of 3% or less. Especially good results were obtained for modeling carbohydrate crystal structures. The fitted crystal structures are a subset of a database of 180 structures systematically selected from the CSD library according to functional group and threshold accuracy criteria. Energy minimization results are also presented for 56 structures not in the training set, which showed cell edge length shifts larger than 3%. The previously published W99 force field, using C(4), C(3), and H(1) potentials, was slightly modified and adopted for hydrocarbon portions of the molecules. Oxygen atoms with one bond, O(1), and those with two bonds, O(2), were assigned separate parameters. Hydrogen atoms bonded to oxygen were assigned exponential repulsion functions and divided into two types: in hydroxy groups, H(2); and in carboxyl groups, H(3). Wavefunctions of HF 6-31g** quality were calculated for every molecule and the molecular electric potential (MEP) was modeled with net atomic charges. Methylene bisector charges were used for all CH2 and CH3 groups, and ring center site charges were added if necessary to fit the MEP. For some structures, upward scaling of the MEP to simulate intermolecular polarization gave better results. MEP interaction generally gave a satisfactory representation of weak CHO hydrogen bonding. Medium strength CHO hydrogen bonding was modeled by reducing the repulsion of the involved hydrogen atoms. Crystal structures of several biologically interesting molecules were modeled with the force field, yielding good results. These molecules include aspirin, sucrose, β-cellobiose, β-lactose, progesterone, testosterone, prostaglandin E1, cholesterol acetate, and stearic acid. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 1–20, 2001