Using a classical potential as an efficient importance function for sampling from an ab initio potential

In this paper the ab initio potential of mean force for the formic acid-wat er system is calculated in a Monte Carlo simulation using a classical fluct uating charge molecular mechanics potential to guide Monte Carlo updates. T he ab initio energies in the simulation are calculated using density-functi onal theory (DFT) methods recently developed by Salahub [J. Chem. Phys. 107 , 6770 (1997)] to describe hydrogen-bonded systems. Importance sampling met hods are used to investigate structural changes and it is demonstrated that using a molecular mechanics importance function can improve the efficiency of a DFT simulation by several orders of magnitude. Monte Carlo simulation of the system in a canonical ensemble at T=300 K reveals two chemical proc esses at intermediate time scales: The rotation of the H2O bonded to HCOOH, which takes place on a time scale of 3 ps, and the dissociation of the com plex which occurs in 24 ps. It is shown that these are the only important s tructural "reactions" in the formic acid-water cluster which take place on a time scale shorter than the double transfer of the proton. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)50936-2].