Effects of the Ewald sum on the free energy of the extended simple point charge model for water Free energy calculations of different phases are necessary to establish the thermodynamically stable phase in simulations. A new method is proposed to calculate the free energy of a crystal of rigid molecules, which is slightly different from the method ͓L. A. Báez and P. Clancy ͑Mol. Phys. 86, 385, ͑1995͔͒. The new method is applied to the ice phase of the TIP4P model for H 2 O ͓W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys. 79, 926 ͑1983͔͒. The free energy of the liquid and gas phase are calculated as well, using different methods as the Widom method, overlapping distribution method, and thermodynamic integration. The melting point of the proton ordered ice Ih of the TIP4P model at atmospheric pressure is found to be T m ϭ214(Ϯ6) K and the boiling point T b ϭ363(Ϯ3) K.

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We describe a method to estimate the critical supersaturation uc above which a growing crystal face becomes kinetically rough. We discuss the relation of uc with the step free energy kTy. The method is tested with computer simulations of the growth of the (001) face of a simple cubic crystal in the solid-on-solid (SOS) model. Near T,, the roughening transition temperature, the results are in good agreement with other estimates of y. The method is then applied to growth of (110) faces of naphthalene from melt and solution. We find that y does not depend on the temperature, in contrast to y found for the SOS model. The (110) face of naphthalene hence does not behave according to the infinite-order roughening transition of Kosterlitz-Thouless type. The classical method to model crystal growth from solution with SOS models turns out to be fundamentally incapable to describe surface roughening in the naphthalene case.

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Simulations of crystal growth from the melt are performed, using the Lennard-Jones potential. Growth of the (100), (111), and (110) faces of the fcc crystal from its melt are simulated. The measured growth rates show that the kinetic coefficient of the (100) face is about twice as high as that of the (111) and (110) faces, which have comparable kinetic coefficients. In order to perform measurements of the interface structure during growth, an order parameter is defined that discriminates between solid and liquid like particles on the basis of the symmetry properties of the local environment. The measurements show that the interface width for the three orientations is similar and does not increase appreciably with undercooling. The difference in occupation fraction in the layers in the interfacial region explains the larger kinetic coefficient found for the (100) face.

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The direct correlation functions and bridge functions for hard spheres near a large hard sphereThe pair correlation function in a homogeneous hard sphere fluid at various densities has been measured in a large system, using the Monte Carlo method. The corresponding direct correlation function C 2 (r) has been determined directly from these measurements, and will be given here in closed form. We conclude that density functional models that neglect the effect of C 3 and higher order direct correlation functions, that are defined in bulk fluids, are not able to describe an inhomogeneous hard sphere fluid accurately.

A new estimation of physical time in Monte Carlo simulations is derived from the requirement that the self-diffusion coefficient measured by Monte Carlo and molecular dynamics simulations have the same value. The dynamics of the particles using both simulation methods are compared by measuring velocity autocorrelation functions. Simulations of pure Lennard-Jones liquids and a binary Lennard-Jones solution show that at small time scales the particle dynamics are different, but at larger time scales they become similar. As a critical test crystal growth from the melt is simulated using the proposed time scale. Both for Monte Carlo and molecular dynamics a linear dependence of the growth rate on undercooling is found and the measured proportionality constant (the kinetic coefficient) is equal to within 6%, i.e., within the statistical error of both methods.