DEVELOPMENT AND TESTING OF THE OPLS ALL-ATOM FORCE-FIELD ON CONFORMATIONAL ENERGETICS AND PROPERTIES OF ORGANIC LIQUIDS

The parametrization and testing of the OPLS all-atom force field for o rganic molecules and peptides are described. Parameters for both torsi onal and nonbonded energetics have been derived, while the bond stretc hing and angle bending parameters have been adopted mostly from the AM BER all-atom force field. The torsional parameters were determined by fitting to rotational energy profiles obtained from ab initio molecula r orbital calculations at the RHF/6-31G//RHF/6-31G* level for more th an 50 organic molecules and ions. The quality of the fits was high wit h average errors for conformational energies of less than 0.2 kcal/mol . The force-field results for molecular structures are also demonstrat ed to closely match the ab initio predictions. The nonbonded parameter s were developed in conjunction with Monte Carlo statistical mechanics simulations by computing thermodynamic and structural properties for 34 pure organic liquids including alkanes, alkenes, alcohols, ethers, acetals, thiols, sulfides, disulfides, aldehydes, ketones, and amides. Average errors in comparison with experimental data are 2% for heats of vaporization and densities. The Monte Carlo simulations included sa mpling all internal and intermolecular degrees of freedom. It is found that such non-polar and monofunctional systems do not show significan t condensed-phase effects on internal energies in going from the gas p hase to the pure liquids.