Nuclear Quantum Effects in Water and Ice

nuclear-quantum-effects-in-ice1

Isotopic Equilibrium volume surface at two different Temperatures. Left T=0K, right T=200 K. Note how different they are. In a normal material they should look very similar.

We are interested in finding different ways of investigating how nuclear quantum effects changes the structure of water and ice.

One of the ways to investigate zero point nuclear quantum effects is to analyse the structural differences of different isotopes. It is experimentally known that in hexagonal ice and water, there is an anomalous isotope effect, where the volume increases when hydrogen is replaced by deuterium. In addition, this anomaly in ice Ih and ice XI increases with temperature and isotope effect is still anomalous in liquid water. We perform free energy calculations with ab initio density functional theory within the quasi-harmonic approximation to explain the origin of this anomaly in hexagonal ice Ih and ice XI. [1] To better understand this anomaly in water, we extend our study to analyse ices and ice-like structures with different densities. We first study the isotope effect in clathrate hydrates and show theoretical calculations agree with the experimental observation of anomalous isotope effects in these systems. Then we analyse the source of normal isotope effect in a very dense phase of ice, ice VIII, which is only stable under pressure. Finally, we investigate isotope effect on amorphous ices, which structurally resemble liquid water. We perform our calculations on amorphous ices with different densities to understand how the isotope effect changes when we have a low or high density liquid like amorphous ice.

Another way to investigate nuclear quantum effects in water is by performing path integral molecular dynamics (PIMD) simulations. Our work involves developing a new thermostating framework, called NGLE, with which we can achieve zero point energy of independent vibrational modes. [2] We are also interested in understanding how PIMD results are dependent on the underlying exchange and correlation functional. Understanding normal-anomalous isotope effects in water can be linked to LDL-like and HDL-like liquid structure of water.

References:
[1] Anomalous Nuclear Quantum Effects in Ice
B. Pamuk, J. M. Soler, R. Ramirez, C. P. Herrero, P. W. Stephens, P. B. Allen, and M.-V. Fernandez-Serra, Phys. Rev. Lett. 108, 193003 (2012).
[2] Simulation of quantum zero-point effects in water using a frequency dependent thermostat
Sriram Ganeshan, R. Ramirez, and M. V. Fernández-Serra, Phys. Rev. B 87, 134207 (2013).