The performance of several popular water models (TIP3P, TIP4P, TIP5P and TIP4P/2005) is analysed.... more The performance of several popular water models (TIP3P, TIP4P, TIP5P and TIP4P/2005) is analysed. For that purpose the predictions for ten different properties of water are investigated, namely: 1. vapour-liquid equilibria (VLE) and critical temperature; 2. surface tension; 3. densities of the different solid structures of water (ices); 4. phase diagram; 5. melting point properties; 6. maximum in density at room pressure and thermal coefficients $\alpha$ and $\kappa_T$; 7. structure of liquid water and ice; 8. equation of state at high pressures; 9. diffusion coefficient; 10. dielectric constant. For each property, the performance of each model is analysed in detail with a critical discussion of the possible reason of the success or failure of the model. A final judgement on the quality of these models is provided. TIP4P/2005 provides the best description of almost all properties of the list, with the only exception of the dielectric constant. In the second position, TIP5P and TIP4P yield an overall similar performance, and the last place with the poorest description of the water properties is provided by TIP3P. The ideas leading to the proposal and design of the TIP4P/2005 are also discussed in detail. TIP4P/2005 is probably close to the best description of water that can be achieved with a non polarizable model described by a single Lennard-Jones (LJ) site and three charges.
The new refrigerant R152a (CH3-CHF2) is modelled as a fluid of homo-nuclear two-centre Lennard-Jo... more The new refrigerant R152a (CH3-CHF2) is modelled as a fluid of homo-nuclear two-centre Lennard-Jones molecules with a point dipole along the axis. This 2CLJD potential has four parameters which were determined by using results of previous and new molecular dynamics ...
We present an application of Wertheim's Thermodynamic Perturbation Theory (TPT1) to a simple coar... more We present an application of Wertheim's Thermodynamic Perturbation Theory (TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones monomers. We use both the Reference Hyper-Netted-Chain (RHNC) and Mean Spherical approximation (MSA) integral equation theories to describe the properties of the reference fluid. The equation of state, the density dependence of the excess chemical potential, and the critical points of the liquid--vapor transition are compared with simulation results and good agreement is found. The RHNC version is somewhat more accurate, while the MSA version has the advantage of being almost analytic. We analyze the scaling behavior of the critical point of chain fluids according to TPT1 and find it to reproduce the mean field exponents: The critical monomer density is predicted to vanish as $n^{-1/2}$ upon increasing the chain length $n$ while the critical temperature is predicted to reach an asymptotic finite temperature that is attained as $n^{-1/2}$. The predicted asymptotic finite critical temperature obtained from the RHNC and MSA versions of TPT1 is found to be in good agreement with the $\Theta$ point of our polymer model as obtained from the temperature dependence of the single chain conformations.
A number of crystal structures of water have been `superheated' in Monte Carlo simulations. Two w... more A number of crystal structures of water have been `superheated' in Monte Carlo simulations. Two well known models for water were considered; namely the TIP4P model and the SPC/E model. By comparing the fluid-solid coexistence temperature to the temperature at which the solid becomes mechanically unstable and melts it is possible to determine the typical range of temperatures over which is possible to superheat the ice phases in conventional simulation studies. It is found that the ice phases can be superheated to approximately 90K beyond the fluid-solid coexistence temperature. Beyond this limit they spontaneously melt. This limit appears to depend weakly both on the type of ice phase considered and on the chosen model. Obviously only rigorous free energy calculations can determine the equilibrium fluid-solid coexistence of a model. However, a "rule of thumb" is that, by subtracting 90K from the mechanically stability limit of the the ice phase one is provided with a first guess as to the equilibrium fluid-solid coexistence temperature.
Over the last forty years many computer simulations of water have been performed using rigid non-... more Over the last forty years many computer simulations of water have been performed using rigid non-polarizable models. Since these models describe water interactions in an approximate way it is evident that they cannot reproduce all of the properties of water. By now many properties for these kinds of models have been determined and it seems useful to compile some of these results and provide a critical view of the successes and failures. In this paper a test is proposed in which 17 properties of water, from the vapour and liquid to the solid phases, are taken into account to evaluate the performance of a water model. A certain number of points between zero (bad agreement) and ten (good agreement) are given for the predictions of each model and property. We applied the test to five rigid non-polarizable models, TIP3P, TIP5P, TIP4P, SPC/E and TIP4P/2005, obtaining an average score of 2.7, 3.7, 4.7, 5.1, and 7.2 respectively. Thus although no model reproduces all properties, some models perform better than others. It is clear that there are limitations for rigid non-polarizable models. Neglecting polarizability prevents an accurate description of virial coefficients, vapour pressures, critical pressure and dielectric constant. Neglecting nuclear quantum effects prevents an accurate description of the structure, the properties of water below 120 K and the heat capacity. It is likely that for rigid non-polarizable models it may not be possible to increase the score in the test proposed here beyond 7.6. To get closer to experiment, incorporating polarization and nuclear quantum effects is absolutely required even though a substantial increase in computer time should be expected. The test proposed here, being quantitative and selecting properties from all phases of water can be useful in the future to identify progress in the modelling of water.
Understanding the critical properties of chain molecules ... By CARLOS VEGA and LUIS G. MACDOWELL... more Understanding the critical properties of chain molecules ... By CARLOS VEGA and LUIS G. MACDOWELL Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain ... The vapour-liquid equilibrium of n-alkanes is ...
A perturbation theory, derived by Boublik, for the polar hard Gaussian overlap model is extended ... more A perturbation theory, derived by Boublik, for the polar hard Gaussian overlap model is extended to other hard polar models. The final equations are simple, and good agreement between theory and simulation is found for quad-rupolar hard dumbbells and for quadrupolar hard ...
The performance of several popular water models (TIP3P, TIP4P, TIP5P and TIP4P/2005) is analysed.... more The performance of several popular water models (TIP3P, TIP4P, TIP5P and TIP4P/2005) is analysed. For that purpose the predictions for ten different properties of water are investigated, namely: 1. vapour-liquid equilibria (VLE) and critical temperature; 2. surface tension; 3. densities of the different solid structures of water (ices); 4. phase diagram; 5. melting point properties; 6. maximum in density at room pressure and thermal coefficients $\alpha$ and $\kappa_T$; 7. structure of liquid water and ice; 8. equation of state at high pressures; 9. diffusion coefficient; 10. dielectric constant. For each property, the performance of each model is analysed in detail with a critical discussion of the possible reason of the success or failure of the model. A final judgement on the quality of these models is provided. TIP4P/2005 provides the best description of almost all properties of the list, with the only exception of the dielectric constant. In the second position, TIP5P and TIP4P yield an overall similar performance, and the last place with the poorest description of the water properties is provided by TIP3P. The ideas leading to the proposal and design of the TIP4P/2005 are also discussed in detail. TIP4P/2005 is probably close to the best description of water that can be achieved with a non polarizable model described by a single Lennard-Jones (LJ) site and three charges.
The new refrigerant R152a (CH3-CHF2) is modelled as a fluid of homo-nuclear two-centre Lennard-Jo... more The new refrigerant R152a (CH3-CHF2) is modelled as a fluid of homo-nuclear two-centre Lennard-Jones molecules with a point dipole along the axis. This 2CLJD potential has four parameters which were determined by using results of previous and new molecular dynamics ...
We present an application of Wertheim's Thermodynamic Perturbation Theory (TPT1) to a simple coar... more We present an application of Wertheim's Thermodynamic Perturbation Theory (TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones monomers. We use both the Reference Hyper-Netted-Chain (RHNC) and Mean Spherical approximation (MSA) integral equation theories to describe the properties of the reference fluid. The equation of state, the density dependence of the excess chemical potential, and the critical points of the liquid--vapor transition are compared with simulation results and good agreement is found. The RHNC version is somewhat more accurate, while the MSA version has the advantage of being almost analytic. We analyze the scaling behavior of the critical point of chain fluids according to TPT1 and find it to reproduce the mean field exponents: The critical monomer density is predicted to vanish as $n^{-1/2}$ upon increasing the chain length $n$ while the critical temperature is predicted to reach an asymptotic finite temperature that is attained as $n^{-1/2}$. The predicted asymptotic finite critical temperature obtained from the RHNC and MSA versions of TPT1 is found to be in good agreement with the $\Theta$ point of our polymer model as obtained from the temperature dependence of the single chain conformations.
A number of crystal structures of water have been `superheated' in Monte Carlo simulations. Two w... more A number of crystal structures of water have been `superheated' in Monte Carlo simulations. Two well known models for water were considered; namely the TIP4P model and the SPC/E model. By comparing the fluid-solid coexistence temperature to the temperature at which the solid becomes mechanically unstable and melts it is possible to determine the typical range of temperatures over which is possible to superheat the ice phases in conventional simulation studies. It is found that the ice phases can be superheated to approximately 90K beyond the fluid-solid coexistence temperature. Beyond this limit they spontaneously melt. This limit appears to depend weakly both on the type of ice phase considered and on the chosen model. Obviously only rigorous free energy calculations can determine the equilibrium fluid-solid coexistence of a model. However, a "rule of thumb" is that, by subtracting 90K from the mechanically stability limit of the the ice phase one is provided with a first guess as to the equilibrium fluid-solid coexistence temperature.
Over the last forty years many computer simulations of water have been performed using rigid non-... more Over the last forty years many computer simulations of water have been performed using rigid non-polarizable models. Since these models describe water interactions in an approximate way it is evident that they cannot reproduce all of the properties of water. By now many properties for these kinds of models have been determined and it seems useful to compile some of these results and provide a critical view of the successes and failures. In this paper a test is proposed in which 17 properties of water, from the vapour and liquid to the solid phases, are taken into account to evaluate the performance of a water model. A certain number of points between zero (bad agreement) and ten (good agreement) are given for the predictions of each model and property. We applied the test to five rigid non-polarizable models, TIP3P, TIP5P, TIP4P, SPC/E and TIP4P/2005, obtaining an average score of 2.7, 3.7, 4.7, 5.1, and 7.2 respectively. Thus although no model reproduces all properties, some models perform better than others. It is clear that there are limitations for rigid non-polarizable models. Neglecting polarizability prevents an accurate description of virial coefficients, vapour pressures, critical pressure and dielectric constant. Neglecting nuclear quantum effects prevents an accurate description of the structure, the properties of water below 120 K and the heat capacity. It is likely that for rigid non-polarizable models it may not be possible to increase the score in the test proposed here beyond 7.6. To get closer to experiment, incorporating polarization and nuclear quantum effects is absolutely required even though a substantial increase in computer time should be expected. The test proposed here, being quantitative and selecting properties from all phases of water can be useful in the future to identify progress in the modelling of water.
Understanding the critical properties of chain molecules ... By CARLOS VEGA and LUIS G. MACDOWELL... more Understanding the critical properties of chain molecules ... By CARLOS VEGA and LUIS G. MACDOWELL Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain ... The vapour-liquid equilibrium of n-alkanes is ...
A perturbation theory, derived by Boublik, for the polar hard Gaussian overlap model is extended ... more A perturbation theory, derived by Boublik, for the polar hard Gaussian overlap model is extended to other hard polar models. The final equations are simple, and good agreement between theory and simulation is found for quad-rupolar hard dumbbells and for quadrupolar hard ...
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