MOLECULAR-DYNAMICS SIMULATION FOR LIQUID WATER USING A POLARIZABLE AND FLEXIBLE POTENTIAL

A molecular dynamics simulation for liquid water at room temperature is reported. A sample of 1000 water molecules has been analyzed with the Nieser-Corongiu-Clementi polarizable potential, to which is added vibrational flexibility. The sample was contained in a cubic box subject to periodic boundary conditions, a cutoff radius of 15.54 angstrom was used in evaluating energies and forces, the long-range interactions were taken into account with the reaction field method, and the integration of the equation of motion has been performed using a sixth-order Gear predictor-corrector algorithm. The simulation, after 35 ps of equilibration time, has been carried out for 10.4 ps with a time step of 1.25 x 10(-16) s. The trajectories and velocities, collected in the production phase of the simulation, have been used to compute the geometry of the water molecule within the liquid, thermodynamical quantities, the pair correlation functions, the Dore neutron pair correlation function, X-ray and neutron scattering structure functions, translational, rotational, orientational autocorrelation functions and their Fourier transforms, mean square displacements, diffusion coefficients, and NMR relaxation times. A detailed study on the spectral density is presented, and good agreement is found with the most recent infrared, Raman, and neutron scattering laboratory data. All the simulated properties so far analyzed are in good agreement with experimental data, and, with confidence, we can assess that our ab initio polarizable potential with inclusion of vibrations is the most reliable water potential in today's literature for the description of water.