(319e) 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).