Comparison of the hybrid AM1/TIP3P and the OPLS functions through Monte Carlo simulations of acetic acid in water

The hybrid quantum mechanical (QM) AM1/TIP3P and the empirical OPLS potential functions have been examined through statistical mechanical Monte Carlo simulations of acetic acid in water at 25°C and 1 atm. It was found that the results obtained from these two approaches are in good accord, supporting the validity of the combined QM and molecular mechanical (MM) approximation in condensed-phase simulations. In the AM1/TIP3P treatment, acetic acid was characterized by the valence electrons and the nucleus cores with the AM1 theory, while the TIP3P model was adopted for the solvent molecules. It was noted that the predicted hydrogen-bonding energies for bimolecular complexes using the AM1/TIP3P potential are in good accord with the ab initio 6-31G(d) results; however, hydrogen-bond distances in the QM hydrogen-bond donor complexes are about 0.2-0.4 Å shorter than those predicted by the empirical function and ab initio calculations. This was further characterized by energy distribution and radical distribution functions obtained from the fluid simulations. In addition, Mulliken population analyses in the Monte Carlo simulations revealed details of the polarization effects on solute charge distribution, leading to large induced dipole moments in aqueous solution.