Filtered stress autocorrelation functions of liquid water models

Abstract

Pressure autocorrelation functions of two models, SPC/E and SPC/Fw, ofpure liquid water are presented. Periodic boundary condition simulations, in themicrocanonical ensemble (NVE), of 256 molecules at room temperature are accomplishedfor both models. Green-Kubo relations are used over the stress tensor time series to extractviscosity properties of the system. Filtering of noise and signal in the numerical data isconsidered. Three steps are discussed to reach relevant physical data pertaining totransport coefficients calculations: (1) removing noise via Savitzky-Golay filters to smoothsignals, (2) fitting data by combining Kohlrausch type functions, (3) separating lowfrequency from high frequency behavior. On the latter resides the essential differencebetween rigid and flexible models. Considerations about the stress tensor structure inflexible case, and the physical meaning each part holds, are explained and used to showsimilarities in low frequency (librational/translational and cluster) motions present in bothmodels.