274629 Exploring the Low-Temperature Phase Behavior of ST2 Water with Well-Tempered Metadynamics
Many of the well-known thermodynamic anomalies of water, such as its negative thermal expansion and increased compressibility upon cooling, become more pronounced when it is cooled below its freezing point into a metastable liquid. On the basis of numerical evidence from molecular simulation of the ST2 water model1, Poole et al.2 proposed that these anomalies may be explained by the presence of a second critical point associated with the coexistence of two distinct metastable liquid phases in the deeply supercooled regime, a low density liquid (LDL) and high density liquid (HDL). Since the hypothesized LDL-HDL coexistence region is well below the homogenous nucleation temperature of bulk water, obtaining direct experimental evidence to either support or falsify its existence remains a significant challenge. Subsequent investigations using molecular simulation techniques have reported conflicting results. Liu et al.3 used histogram reweighting methods in the grand canonical ensemble to examine the low-temperature behavior of the ST2 model and found evidence for the existence of two metastable liquid phases. Sciortino et al.4 also reported similar results for the ST2 model, using umbrella sampling grand canonical Monte Carlo simulations with a different treatment of long-ranged electrostatic contributions. However, Limmer and Chandler5 recently used two-dimensional umbrella sampling and histogram reweighting to examine the underlying free energy surface of ST2 water in density and bond-orientational order space, and concluded that one of the observed phases in coexistence is not a liquid but in fact a crystal. We present findings from our recent study of the low temperature phase behavior of ST2 water using the well-tempered metadynamics technique of Parrinello and co-workers6. We discuss the method in detail and demonstrate that it is able to provide reliable and accurate estimates of the free energy surface of ST2 in density and bond-orientational order space in a fraction of the time required by other methods such as umbrella sampling. In agreement with Liu et al.3 and Sciortino et al.4, we show that the free energy surface is comprised of two distinct basins, both of which correspond to liquid-like phases with low bond-orientational order.
1F.H. Stillinger and A. Rahman, J. Chem. Phys., 60, 1545 (1974).
2P. H. Poole, F. Sciortino, U. Essmann, and H. E. Stanley, Nature, 360, 324 (1992)
3Y. Liu, A.Z. Panagiotopoulos and P.G. Debenedetti, J. Chem. Phys., 131, 104508 (2009).
4Sciortino, I. Saika-Voivod and P.H. Poole, Phys. Chem. Chem. Phys., 13, 19759 (2011).
5D. Limmer and D. Chandler, J. Chem. Phys., 135, 134503 (2011)
6A. Barducci, G. Bussi and M. Parrinello, Phys. Rev. Lett., 100, 020603 (2008).