Zhen-Long Lv

Ab initio investigation of the lower energy candidate structures for (H2O)5+ water cluster

The particle swarm optimization method in conjunction with density functional calculations is used to search the lower energy structures for the cationic water clusters (H2O)5(+). Geometry optimization, vibrational analysis, and infrared spectrum calculation are performed for the most interesting clusters at the MP2/aug-cc-pVDZ level. The relationships between their structural arrangements and their energies are discussed. According to their relative Gibbs free energies, their energy order is determined and four lowest energy isomers are found to have a relative population surpassing 1% below 350 K. Studies reveal that, among these four isomers, one new cluster found here also contributes a lot to the experimental infrared spectrum. Based on topological analysis and reduced density gradient analysis, some meaningful points are found by studying the structural characteristics and the bonding strengths of these cationic water clusters: in the first solvation shell, the central H3O(+) motifs may have a stronger interaction with the OH radical than with the water molecules. The interaction in the second solvation shell may also be stronger than that in the first solvation shell, which is opposite to our intuition.

PHASE TRANSITION AND THERMODYNAMIC PROPERTIES OF MAGNESIUM FLUORIDE BY FIRST PRINCIPLES

The structural stabilities, phase transitions and thermodynamic properties of MgF2 under high pressure and temperature are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the local density approximation. The calculated lattice parameters of MgF2 in all four phases under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical results. Our results demonstrate that MgF2 undergoes a series of structural phase transitions from rutile (P42/mnm)→CaCl2-type (Pnnm)→modified fluorite (Pa-3)→cotunnite (Pnam) under high pressure and the obtained transition pressures are in fairly good agreement with the experimental results. The temperature-dependent volume and thermodynamic properties of MgF2 in the rutile phase at 0 GPa are presented and the thermodynamic properties of MgF2 in the rutile, CaCl2-type, modified fluorite and cotunnite phases at 300 K are also predicted using the quasi-harmonic approximation model (QHA) and the quasi-harmonic Debye model (QHD), respectively. Moreover, the partial density of states and the electronic density of the four phases under the phase transition are also investigated.