Derivation of class II force fields. 7. Nonbonded force field parameters for organic compounds

A set of nonbonded force field parameters consisting of atomic partial char ges and van der Waals parameters has been derived by fitting experimental d ata for a broad set of organic compounds. The compounds in the fit spanned 11 functional groups: alcohols, aldehydes, amides, amines, carboxylic acids , esters, ethers, N-heterocycles, hydrocarbons, ketones, and sulfur compoun ds. The data consist of 136 crystal structures, 34 sublimation energies, an d 63 gas-phase dipole moments. The nonbonded potential function is of the " 9-6" form and is used in our class II CFF force field. This paper describes the fitting procedure and the quality of the fit as measured by comparing computed and experimental gas-phase dipole moments and crystal lattice ener gies. As a further test, each crystal structure was optimized. We report th e accuracy of the computed crystal structures, including results obtained w ith force fields derived using two different combination rules. A summary i s presented of the accuracy and consistency of predicted crystal lattice ve ctors and the lattice energies at the computed crystal structures. The resu lts are discussed separately for each of the functional groups.