(159f) The Putative Liquid-Liquid Transition Is a Liquid-Liquid Transition in Some Atomistic Models of Water

Authors: 
Palmer, J., University of Houston
Numerous computational studies by independent research groups have provided strong numerical evidence indicating that the ST2 water model exhibits a metastable liquid-liquid phase transition (LLPT) under deeply supercooled conditions. A similar LLPT has been hypothesized to occur in real water to provide a thermodynamically consistent explanation for its anomalous behaviors, such as its rapid increase in compressibility upon cooling in the supercooled regime. The existence of an LLPT in ST2 and other models of water has been challenged by Limmer and Chandler [J. Chem. Phys. 135, 134503 (2011); J. Chem. Phys. 138, 214504 (2013)], however, who observed signatures of only a single liquid phase in free energy calculations performed near ST2’s LLPT. These findings fueled vigorous debate over the phase behavior of supercooled ST2 water, which has been ongoing for nearly a decade. Although recent studies have provided unambiguous evidence to confirm the existence of a LLPT in ST2, the origin of the discrepancy with the Limmer-Chandler (LC) studies remains unresolved.

Here, we provide a resolution to this long-standing controversy by analyzing the hybrid Monte Carlo (HMC) code that Limmer and Chandler used to perform their free energy calculations for ST2. We show that the LC HMC code suffers from a fundamental design flaw that introduces significant sampling errors. These errors distort ST2’s equation of state and are easily detected using three rigorous statistical mechanics based consistency checks. When we correct the LC code, we recover the correct equation of state of ST2, in excellent agreement with two independent codes; the corrected code also satisfies the consistency checks. The corrected code gives clear evidence of two liquid phases in the free energy surface, while the uncorrected code reproduces the LC published results, which show a single liquid basin at conditions where several other studies have reported two liquid phases.