Ab initio Monte Carlo simulation using multicanonical algorithm: temperature dependence of the average structure of water dimer

A multicanonical Monte Carlo simulation has been performed to investigate a verage structural changes in the water dimer with temperature. Potential en ergy was calculated by the ab initio MO method at the RHF/6-31G* level at e ach step of the simulation. By combining the reweighting technique with the multicanonical algorithm various thermodynamic properties have been evalua ted over a wide range of temperature from a single production run. The tech nique is an ideal tool for studying temperature dependence properties, and the temperature dependence of the distribution functions of various geometr ic parameters has been examined in detail. It was found that the probabilit y of finding structures similar to that of the transition state increases a s the temperature is increased.