A relatively simple equation of state is proposed for several forms of ice, whose parameters have been fitted to the results of extensive computer simulations using the TIP4P/Ice and TIP4P/2005 models of water. Comparison with available experimental data for ice Ih shows that both models reproduce the experimental density and isothermal compressibility to good accuracy over the entire range of thermodynamic stability except at low temperatures. The predictions for the thermal expansion coefficient are slightly worse but still reasonable. Results obtained with the TIP4P/2005 model are slightly better than those obtained with the TIP4P/Ice model. At temperatures below 150 K, the predictions of both models deviate significantly from experiment. As expected, at low temperatures, quantum effects become increasingly important, and classical simulations are unable to accurately describe the properties of ices. In fact, neither the heat capacity nor the thermal expansion coefficient go to zero at zero temperature (as they should be according to the third law of thermodynamics). Predicted compressibilites are however reliable even up to 0 K. Finally, the relative energies of the ices at 0 K have also been estimated and compared with the experiments.

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The second virial coefficients of two-center Lennard-Jones molecules which contain an embedded point dipole have been determined via numerical integration. A number of models with different reduced bond lengths and dipole moments have been considered. For each model the second virial coefficient has been calculated for a number of temperatures. It is shown that the presence of the dipole moment significantly raises the Boyle temperature and, for a given temperature, reduces the value of the second virial coefficient with respect to the non-polar model. It is shown that the model can describe correctly the second virial coefficient of some refrigerants.y Electronic supplementary information (ESI) available: computer programs and ancillary data. See

In this work we combine two recently proposed computer simulation techniques: the wandering interface method for the calculation of surface tensions and Janecek’s method for the estimation of long-range dispersive interactions to predict the surface tension of ethane. We have obtained results for two well-known models, the transferable potentials for phase equilibria (TraPPE) and optimized potentials for liquid simulations (OPLS) force fields. We show that neglecting long-range contributions to the dispersive energy as it is often done may yield surface tensions that are far too low by as much as 40 %. Such contributions can be effectively accounted for, however, and the resulting liquid−vapor coexistence densities and surface tensions show a rather good agreement for the TraPPE model, but quite less so for the OPLS model.

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We present results for the fourth virial coefficient of quadrupolar Lennard-Jones diatomics for several quadrupole moments and elongations. The coefficients are employed to predict the critical properties from two different truncated virial series. The first one employs the exact second and third virial coefficients, calculated in our previous work. The second includes also the exact fourth virial coefficient as obtained in this work. It is found that the first method yields already fairly good predictions. The second method significantly improves on the first one, however, yielding good results for both the critical temperature and pressure. Particularly, when compared with predictions from perturbation theories available in the literature, the virial series to fourth order compares favorably for the critical temperature. The results suggest that the failure of perturbation theories to predict the critical temperature and pressure is not only related to the neglect of density fluctuations, but also to poor prediction of the virial coefficients.

We report numerical results for the third virial coefficient of two center Lennard-Jones quadrupolar molecules. Calculations are performed for 35 models with different elongations and quadrupoles over a temperature range from half to twice the critical temperature. It is found that increasing the elongation at fixed quadrupole has the effect of increasing B 3 . On the other hand, at fixed elongation B 3 first decreases with increasing quadrupole at low temperatures, then increases with increasing quadrupole at higher temperatures. We estimate the temperature at which the third virial coefficient vanishes. Although both this temperature and the critical temperature increase with the quadrupole moment, their ratio remains almost constant. We predict the critical properties using two different truncated virial series. The first one employs the exact second and third virial coefficients. The second one approximates the fourth order contribution by using estimates obtained for hard diatomics. It is found that both methods yield fairly good predictions, with a somewhat better performance of the approximate fourth order expansion. The two methods are complementary, however, because they consistently bracket the exact value as determined from computer simulations.