An efficient quantum mechanical/molecular mechanics Monte Carlo simulation of liquid water

An efficient implementation of a quantum mechanical/molecular mechanics (QM/MM) Monte Carlo (MC) simulation of liquid water is presented. The auto-correlation function of the energy is used to reduce the number of QM calculations during the simulation. The effects of the liquid structure on the dipole moment of the quantum water molecule is considered. Using only 80 QM calculations at the Hartree–Fock/6-31++G(d,p) level but on statistically nearly uncorrelated structures, we find an average dipole moment for the liquid water of 2.73 ± 0.13 D. This corresponds to an induced dipole moment of 0.44 ± 0.13 D on going from the gas phase to the liquid phase, in excellent agreement with previous, more extensive computational procedures.