Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids

TLDR

The study develops and evaluates an OPLS all‑atom force field for organic molecules and peptides. Parameters were derived from ab initio rotational energy profiles for torsional terms, borrowed AMBER bond and angle terms, and optimized nonbonded interactions through Monte Carlo simulations of 34 pure organic liquids, sampling all internal and intermolecular degrees of freedom. The force field achieves excellent agreement with ab initio conformational energies (≤0.2 kcal/mol) and structures, reproduces experimental heats of vaporization and densities within 2 %, and shows negligible condensed‑phase effects on internal energies for non‑polar, monofunctional systems.

Abstract

The parametrization and testing of the OPLS all-atom force field for organic molecules and peptides are described. Parameters for both torsional and nonbonded energetics have been derived, while the bond stretching and angle bending parameters have been adopted mostly from the AMBER all-atom force field. The torsional parameters were determined by fitting to rotational energy profiles obtained from ab initio molecular orbital calculations at the RHF/6-31G*//RHF/6-31G* level for more than 50 organic molecules and ions. The quality of the fits was high with average errors for conformational energies of less than 0.2 kcal/mol. The force-field results for molecular structures are also demonstrated to closely match the ab initio predictions. The nonbonded parameters 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 sampling all internal and intermolecular degrees of freedom. It is found that such non-polar and monofunctional systems do not show significant condensed-phase effects on internal energies in going from the gas phase to the pure liquids.