Page 268 - Handbook Of Multiphase Flow Assurance

Molecular modeling 267

Analysis of the water structure for different models
After the runs were finished, an ASCII file with coordinates of particles in time was gen-
erated and processed by a Fortran program (see Computer Code section) to obtain the oxy-
gen- oxygen radial distribution function (RDF). RDF is an indicator of density fluctuations of
system. RDF for three water models were compared against X-ray diffraction experimental
data (Narten and Lewy, 1971). Experimental data are available for water at a density (1.01 g/
cc) and temperature (298 K) which is close to the simulation conditions. Comparison shows
that SPC and TIP3p models are in good agreement with experiment whereas the Tripos model
doesn't represent the water structure very well.
Runs for SPC and TIP3p water were continued up to 50,000 fs (50,000 time steps for TIP3p
water and 76,923 time steps for SPC water). This difference is explained by 1 fs time step for
TIP3p model and 0.65 fs time step required for SPC water model. RDF for these runs were
generated and are presented in Figs. 10.37 and 10.38. After the SYBYL® implementation of

FIG. 10.37 RDF oxygen-oxygen for TIP3p model and water at 298 K. ***, Narten and Lewy (1971), _ _ _ , this work.