FLEXIBLE MOLECULAR-MODELS FOR MOLECULAR-DYNAMICS STUDY OF NEAR AND SUPERCRITICAL WATER

Four flexible water models based an the harmonic and a modified anharm onic form of Toukan and Rahman intramolecular potential models grafted onto 3-site and 4-site intermolecular models were proposed, and the i nfluence of their flexibility and intermolecular potential on the phas e behavior were examined. A global optimization of the potential param eters was performed according to the physical properties and IR spectr a of water at the ambient state. The critical properties of the four m odels were estimated by using direct molecular dynamics (MD) simulatio n of liquid-vapor coexisting phases and fitting the coexisting densiti es to the law of rectilinear diameter and the scaling law for density with a scaling exponent of beta(c) = 0.325. Based on a similar intermo lecular model (the simple point charge model, SPC) we found that the a nharmonic model gave a critical point (T-c = 370.3 +/- 3.6 degrees C; rho(c) = 0.32 +/- 0.006 g/cm(3)) which agreed well with the experiment al properties (T-c = 374.0 degrees C; rho(c) = 0.322 g/cm(3)), while t he harmonic model gave low values (T-c = 337.3 +/- 8.5 degrees C; rho( c) = 0.290 +/- 0.007 g/cm(3)). On the other hand, for anharmonic flexi ble water models adopting the experimental value, the model grafted on to a 3-site TIPS model (the transferable intermolecular potential func tions, TIP3P) did not provide satisfactory reproduction of the critica l point (T-c = 320.1 +/- 5.2 degrees C; rho(c) = 0.288 +/- 0.005 g/cm( 3)), while the model grafted onto a 4-site TIPS model (modified TIP4P model with M-site on the center of mass, referred to as cm4P) provided good representation of critical properties (T-c = 368.4 +/- 6.2 degre es C; rho(c) = 0.307 +/- 0.005 g/cm(3)). The flexible models with anha rmonic form of flexibility grafted the SPC or cm4P model are suitable model for study of water and aqueous solutions in state points ranging from liquid to near- and supercritical region. (C) 1998 Elsevier Scie nce B.V.