The effect of intra-molecular bonds on the liquid-liquid critical point in modified-WAC models.

To obtain a better understanding of liquid-liquid critical points (LLCPs) in one-component liquids, we extend the modified-WAC model by E. Lascaris, Phys. Rev. Lett. 116, 125701 (2016) which is known to have a LLCP. The original WAC model is a model for silica (SiO2) and consists of a mixture of non-bonded Si and O ions. By adding explicit intra-molecular Si-O bonds to the model, we are able to study how several parameters (Si-O bond length, O-Si-O angle, and bond stiffness) affect the existence and location of the LLCP. We find that for this model, only the Si-O bond length has a strong effect on the LLCP, while the bond angle and bond stiffness have no significant effect on the LLCP. An analysis of the relevant coordination numbers indicates that increasing the bond length decreases the ratio RSi/O of additional Si ions per additional O ion in the first coordination shell of the Si, which causes the LLCP to move to higher, more accessible temperatures. The behavior of the RSi/O parameter shows a strong correlation with the behavior of the LLCP and might be a useful tool to determine if a LLCP exists at low, hard-to-reach temperatures in other models.

Finite-size scaling investigation of the liquid-liquid critical point in ST2 water and its stability with respect to crystallization.

The liquid-liquid critical point scenario of water hypothesizes the existence of two metastable liquid phases--low-density liquid (LDL) and high-density liquid (HDL)--deep within the supercooled region. The hypothesis originates from computer simulations of the ST2 water model, but the stability of the LDL phase with respect to the crystal is still being debated. We simulate supercooled ST2 water at constant pressure, constant temperature, and constant number of molecules N for N ≤ 729 and times up to 1 µs. We observe clear differences between the two liquids, both structural and dynamical. Using several methods, including finite-size scaling, we confirm the presence of a liquid-liquid phase transition ending in a critical point. We find that the LDL is stable with respect to the crystal in 98% of our runs (we perform 372 runs for LDL or LDL-like states), and in 100% of our runs for the two largest system sizes (N = 512 and 729, for which we perform 136 runs for LDL or LDL-like states). In all these runs, tiny crystallites grow and then melt within 1 µs. Only for N ≤ 343 we observe six events (over 236 runs for LDL or LDL-like states) of spontaneous crystallization after crystallites reach an estimated critical size of about 70 ± 10 molecules.