Free energy of liquid water from a computer simulation via cell theory

摘要

A method to calculate the free energy of water from computer simulation is presented.Based on cell theory,it approximates the potential energy surface sampled in the simulation by an anisotropic six-dimensional harmonic potential to model the three hindered translations and three hindered rotations of a single rigid water molecule.The potential is parametrized from the magnitude of the forces and torques measured in the simulation.The entropy of these six harmonic oscillators is calculated and summed with a conformational term to give the total entropy.Combining this with the simulation enthalpy yields the free energy.The six water models examined are TIP3P,SPC,TIP4P,SPC/E,T1P5P,and TIP4P-Ew.The results reproduce experiment well:free energies for all models are within 1.6 kJ mol-1 and entropies are within 3.6 J K-1 mol-1.Approximately two-thirds of the entropy comes from translation,a third from rotation,and 5% from conformation.Vibrational frequencies match those in the experimental infrared spectrum and assist in their assignment.Intermolecular quantum effects are found to be small,with free energies for the classical oscillator lying 0.5-0.7 kJ mol-1 higher than in the quantum case Molecular displacements and vibrational and zero point energies are also calculated.Altogether,these results validate the harmonic oscillator as a quantitative model for the liquid state.