Liang-Chun Liu

    Water is important in biological functions and receives much attention in the science community. Though being very much studied, its physical and chemical properties as well as the origins of many water anomalies are not yet fully understood. It is conjectured that the anomalies (such as density, structure, thermodynamic, and physical properties) may be explained through the two-fluid model, namely, two types of structurally different water, the high density liquid (HDL) and the low density liquid (LDL), coexist in the supercooled regime. However, experimental study to the existence of the two fluids is actually quite hindered because when decreasing the temperature, system enters crystallization stage earlier than the supercooled regime, which makes the experimental study of the supercooled fluid not approachable.

Numerically, it is possible to investigate the aforementioned fluid states since crystallization requires a longer time scale than what can be done in the current computational capability. Aligned with this approach, the very first step is to retrieve an accurate phase diagram. With the correct topologies at different states, it is then possible to collect the thermodynamic and dynamic properties. However, it is not easy even for the first step since the system quickly enters the glassy state when temperature is decreased. In the language of potential energy landscape, it means that the kinetic energy is too low to help the system to escape from some local potential wells. The relaxation for these trapped states may takes up to several seconds, which is still not easily attainable with the current computational ability. Hence, various accelerated molecular dynamics (MD) methods have been developed to tackle this slow dynamics problem. Here we used the volume-density replica exchange molecular dynamics (VTREMD) to compute the water phase diagram which spans a very large scale phase space. The aforementioned method is one of the accelerated MD, which is accomplished by an enhanced sampling of states. This study constructs a system of 720 replicas of TIP4P/2005 water molecules in each one. The temperature grid spans from 145 K to 363 K, where the low temperature end is hardly achievable via classical molecular dynamics. A typical VTREMD test required 720 cores running in parallel for at least 20 ns simulation time, namely, 144,000 CPU hours. With this large-scale test, system convergence is speeded up (from possibly several ms to a dozen of ns) and thus renders a possible investigation to the deeply supercooled water. Next, LDL-like and HDL-like molecules are labelled by its distance of the 5th nearest neighbor oxygen, where those with distance larger (smaller) than 0.35 nm are LDL-like (HDL-like) ones. This study reveals the distinct behaviour of the two-fluid mixture under different states and is helpful in explaining the origin of water anomalies.