Applications of path-integral molecular dynamics simulations to phase transition problems

Abstract

In order to predict the properties of materials from first principles simulations, both the electronic structures and the movement of nuclei should be accurately described. Till now, the electronic structure calculation methods have been relatively well developed, like the density-functional theory, the quantum chemical methods, and the diffusion quantum monte carlo method, etc. By using them, we can get the electronic structures of materials with different levels of accuracy. But the movements of nuclei are still often treated at a clasical level. More and more experimental and theoretical works have found that, the nuclear quantum effects (NQEs) are playing an important role in some phase transition porcesses. Melting line of hydrogen at high pressures were reported through coexistance simulations based on ab initio PIMD methods. We determined the melting temperature as a function of pressure and finded an atomic solid phase from 500 to 800 GPa, which melts at less than 200 K. Beyond this and up to 1200 GPa, a metallic atomic liquid is stable at temperatures as low as 50 K. The quantum motion of the protons is the key to the low melting temperature reported, as simulations with classical nuclei lead to considerably higher melting temperatures of about 300 K across the entire pressure range considered. Then, a combination of state-of-the-art theoretical methods were used to obtain an atomic level picture of classical and quantum ordering of protons in cold high-pressure solid hydrogen. Besides, Using a self-developed combination of the thermodynamic integration and the ab initio PIMD methods, we quantitatively studied the influence of NQEs on the melting of dense lithium at 45 GPa. We find that although the NQEs significantly change the free-energies of the competing solid and liquid phases, the melting temperature is lowered by only about 15 K, with values obtained using both classical and quantum nuclei in close proximity to a new experiment. So, by combining the path-integral molecular dynamics (PIMD) method with techniques like coexistance simulation and free-energy calculation method, the NQEs in many phase transition processes can be well understood.