Determining the Phase Behavior of polarizable Water Models
EMSL Project ID
3104a
Abstract
Molecular simulation is used as a tool in the understanding of macroscopic observations from microscopic events. However, the results of a simulation are dependent on the underlying potential functions that describe interactions between chemical species. For the case of water, it has been shown that inclusion of only two-body interactions as determined from the corresponding dimer PES leads to erroneous results in the predicted phase behavior because many-body non-additive effects are also important.1 Water has been extensively studied using molecular simulation due to its relevance in a variety of fields. Recently, a new empirical interaction potential has been developed by Burnham and Xantheas2 from the results of high level electronic structure calculations on small water clusters. This model, TTM2-R, is an all-atom polarizable, rigid model, accurately reproducing the binding energy in small clusters and several bulk properties. The model?s ability to reproduce the phase behavior of water has not yet been determined. Due to its formulation, the TTM2-R model is more computationally expensive than a pairwise-additive model, especially when implemented in a Monte Carlo simulation.2 It is proposed to determine the phase coexistence properties of water using the TTM2-R potential and sophisticated Monte Carlo algorithms that sample the free energy as a function of temperature and density. The phase behavior of the TTM2-R water model will be obtained from Gibbs ensemble Monte Carlo 3 and grand canonical Monte Carlo (GCMC) simulations with histogram reweighting.4 The advantage of using GCMC with histogram reweighting is that fewer simulations are needed to produce the phase envelope as long as accurate simulations around the critical point are performed. Due to the computational cost of calculating the system energy at every step, as specified in the model, a small system (500 ?3) will be considered. Our preliminary calculations have shown that in order to get good sampling for a single data point, 150 CPU hours are necessary. In order to generate the phase envelope for the TTM2-R model, approximately 200 simulation points are necessary, corresponding to 30,000 node-hours. The results of this study will be used to compare the TTM2-R model with existing water potentials in the prediction of the phase behavior. We will furthermore explore the possibility of using the flexible version of the developed model, TTM2-F, in order to assess the effect of intramolecular motion on the vapor liquid equilibrium. This will require an additional 20,000 node-hours, for a total requested allocation of 50,000 node-hours. This work is an initial step in developing a methodology for using ab initio data in determining force field parameters for accurate computer simulations of thermodynamic and phase equilibrium properties for wide classes of chemical species. It is of interest to identify which microscopic properties and to what extend control the macroscopic ones such as the critical density and temperature and attempt a further fine-tuning of the model based on these results.
The proposed research effort is a collaboration between the group of Prof. Panagiotopoulos at Princeton University and Dr. Sotiris Xantheas of PNNL.
1S. S. Xantheas, J. Chem. Phys. 100, 7523 (1994); S. S. Xantheas, Philos. Mag. B 73, 107 (1996); M. P. Hodges and A. J. Stone, S. S. Xantheas, J. Phys. Chem. A 101, 9163 (1997); S. S. Xantheas, Chem. Phys. 258, 225 (2000); M. Hloucha, A. K. Sum, and S. I. Sandler, J. Chem. Phys. 113, 5401 (2000).
2 C. J. Burnham and J. Li, S. S. Xantheas, M. Leslie, J. Chem. Phys. 110, 4566 (1999); C. J. Burnham, and S. S. Xantheas, J. Chem. Phys. 116, 1500 (2002); ibid. 116, 5115 (2002).
3A. Z. Panagiotopoulos, Mol. Phys. 61, 813 (1987).
4A. M. Ferrenberg, and R. H. Swendsen, Phys. Rev. Lett. 61, 2635 (1988).
Project Details
Project type
Exploratory Research
Start Date
2003-12-09
End Date
2004-12-28
Status
Closed
Released Data Link
Team
Principal Investigator