Isothermal isobaric molecular dynamics simulation of water

Abstract

A series of isothermal isobaric molecular dynamics simulation are carried out on a system made up of 210 molecules at normal temperature and pressure ( T = 298 K, P = 1 bar) and at high pressure ( T = 298 K, P = 2.1 kbar). The physical model is based upon the tetrahedral cluster architecture composed of one central molecule (20% of the sample) and four outer molecules (80%). This theoretical approach of the liquid water structure and description was recently strengthened by spectroscopy measurements (Raman spectrometry, X-ray and neutron scattering) on the three-dimensional network of hydrogen bond at the femto second time scale [Ph. Wernet, D. Nordlund, U. Bergmann, M. Cavalleri, M. Odelius, H. Ogasawara, L.A. Naslund, T.K. Hirsch, L. Ojamae, P. Glatzel, L.G.M. Pettersson, A. Nilsson, Science 304 (2004) 995; Jared D. Smith, Christopher D. Cappa, Benjamin M. Messer, Walter S. Drisdell, Ronald C. Cohen, Richard J. SayKally, J. Phys. Chem. B 110 (2006) 20038; Jared D. Smith, Christopher D. Cappa, Kevin R. Wilson, Ronald C. Cohen, Phillip L. Geissler, Science 306 (2004)]. This work corroborates that the number of water molecules having two and four hydrogen bonds in liquid water represents the relative molecular composition of 20% and 80%, respectively, and improves our previous mixture model [M. Amrani, W. Sellaoui, M. Belhakem, A. Krallafa, D. Bormann, J. Soc. Algiers Chim. 13(2) (2003) 227–237] of water, focusing in particular on the charge asymmetry and distribution of water molecules within the liquid. For that purpose, investigations of the influence of the charge delocalization on the central molecule of our pentamer upon various calculated data are performed. A high pressure is applied to highlight optimal selective polarization on the hydrogen bond network forming the Walrafen’s cluster [D. Eizenberg, W. Kauzmann, The Structure and the Properties of Water, Springer, 1968] at room temperature. A complementary series of molecular dynamics simulation are carried out in the temperature range T = 270–285 K under atmospheric pressure for the optimal charge delocalisation and results are compared to the experimental data. Data reported in the present work such as structural data (molecular density or lacunar index, pair distribution function, hydrogen bond arrangements…), transport and thermodynamic properties are found to be in very good agreement with experimental data.