Role of electronic correlation in high-low temperature phase transition of hexagonal nickel sulfide: a comparative density functional theory study with and without correction for on-site Coulomb interaction.

The structural, electronic, magnetic, and elastic properties of hexagonal nickel sulfide (NiS) have been investigated comparatively by Density Functional theory (DFT) and DFT plus correction for on-site Coulomb interaction (DFT+U), in which two different exchange correlation functionals local density approximations (LDA) and general gradient approximations (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) are used. Our results indicate LDA and PBE methods predict hexagonal NiS to be a paramagnetic metal whereas LDA(PBE)+U calculations with reasonable on-site Coulomb interaction energy give the antiferromagnetic insulating state of low temperature hexagonal NiS successfully. Meanwhile, compared with LDA(PBE) results, LDA(PBE)+U methods give larger lattice parameters, crystal volume, and shear constant c44, consistent with the experimental picture during high-low temperature phase transition of hexagonal NiS, in which an increase of the shear constant c44 and lattice parameters were found in the low-temperature antiferromagnetic phase. The present DFT and DFT+U calculations provide a reasonable description for the properties of high temperature and low temperature hexagonal NiS respectively, which indicates that electronic correlation is responsible for this high-low temperature phase transition.

Stability of the polar NiO(111) surface.

Based on density functional theory and thermodynamic model, we compile a phase diagram for the polar NiO(111) surface as a function of temperature and oxygen pressure. The electronic correlation between Ni-3d electrons has also been included in the form of GGA+U method. Consistent with recent experiments, present GGA+U calculation indicates that over a broad range of oxygen partial pressure, the most stable phases are the oxygen and Ni terminated octopolar structures, which are almost degenerate in energy. We also show that the stabilization of the NiO(111) surface goes together with remarkable changes in the geometrical and electronic structure.

Quantum scattering calculation of the O(1D)+HBr reaction.

Three-dimensional time-dependent quantum wave packet calculation for the O((1)D)+HBr reaction has been carried out using an accurate ab initio global potential energy surface [K. A. Peterson, J. Chem. Phys. 113, 4598 (2000)]. The calculations show that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The energy dependence of the reaction cross section is computed, which manifests still resonance structures, and is a decreasing function of the translational energy. The thermal rate constants are also computed, which are nearly independent on the temperature. The calculation results are discussed and compared to similar reaction with deep well.