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We present an accurate computational study of the electronic structure and lattice dynamics of solid molecular hydrogen at high pressure. The band-gap energies of the $C2/c$, $Pc$, and $P6_3/m$ structures at pressures of 250, 300, and 350 GPa are calculated using the diffusion quantum Monte Carlo (DMC) method. The atomic configurations are obtained from ab-initio path-integral molecular dynamics (PIMD) simulations at 300 K and 300 GPa to investigate the impact of zero-point energy and temperature-induced motion of the protons including anharmonic effects. We find that finite temperature and nuclear quantum effects reduce the band-gaps substantially, leading to metallization of the $C2/c$ and $Pc$ phases via band overlap; the effect on the band-gap of the $P6_3/m$ structure is less pronounced. Our combined DMC-PIMD simulations predict that there are no excitonic or quasiparticle energy gaps for the $C2/c$ and $Pc$ phases at 300 GPa and 300 K. Our results also indicate a strong correlation between the band-gap energy and vibron modes. This strong coupling induces a band-gap reduction of more than 2.46 eV in high-pressure solid molecular hydrogen. Comparing our DMC-PIMD with experimental results available, we conclude that none of the structures proposed is a good candidate for phases III and IV of solid hydrogen., Comment: Accepted for publication in Journal of Computational Chemistry. arXiv admin note: text overlap with arXiv:1608.00754

We study the potential energy surface of the ozone molecule by means of Quantum Monte Carlo simulations based on the resonating valence bond concept. The trial wave function consists of an antisymmetrized geminal power arranged in a single-determinant that is multiplied by a Jastrow correlation factor. Whereas the determinantal part incorporates static correlation effects, the augmented real-space correlation factor accounts for the dynamics electron correlation. The accuracy of this approach is demonstrated by computing the potential energy surface for the ozone molecule in three vibrational states: symmetric, asymmetric and scissoring. We find that the employed wave function provides a detailed description of rather strongly-correlated multi-reference systems, which is in quantitative agreement with experiment., Comment: 5 page, 3 figures

We present theoretical calculations of the Raman and IR spectra, as well as electronic properties at zero and finite temperature to elucidate the crystal structure of phase III of solid molecular hydrogen. We find that anharmonic finite temperature are particularly important and qualitatively influences the main conclusions. While P6$_3$/m is the most likely candidate for phase III at the nuclear ground state, at finite temperature the C2/c structure appears to be more suitable., Comment: 7 pages, 6 figures

We investigate the effects of point charge defects on the single particle electronic structure, emission energies, fine structure splitting and oscillator strengths of excitonic transitions in strained In$_{0.6}$Ga$_{0.4}$As/GaAs and strain-free GaAs/Al$_{0.3}$Ga$_{0.7}$As quantum dots. We find that the charged defects significantly modify the single particle electronic structure and excitonic spectra in both strained and strain-free structures. However, the excitonic fine structure splitting, polarization anisotropy and polarization direction in strained quantum dots remain nearly unaffected, while significant changes are observed for strain-free quantum dots., Comment: Submitted to PRB

The sample dependence of various properties of hydrogenated amorphous silicon ($a$-Si:H) have been studied with 216 silicon atoms and 24 hydrogen atoms using the density functional based tight binding molecular dynamics simulations. The Si-Si and Si-H pair correlation functions are independent of preparation procedure as well as initial conditions, the H-H pair correlation functions are sample dependent. The distribution of hydrogen atoms in all the samples is nonuniform and depends upon the preparation procedure as well as the initial structure from which the hydrogenated amorphous silicon sample is generated. The Si-Si bond length and Si-Si-Si bond angle distributions are nearly independent of sample preparation procedure, but Si-H bond length distributions are sample dependent. The peaks in the vibrational density of states (VDOS) at high frequencies are in reasonable accord with the experimental results. The positions of the high frequency peaks are found to be dependent on the local environment. While the high frequency vibrational modes related to Si-Si bond vibrations are moderately localized, the vibrational modes related to Si-H bond vibrations in $a$-Si:H samples are highly localized. In samples generated from the liquid quench the free energy is smaller, whereas the entropy and specific heat are larger as compared to that of samples generated by hydrogenation of pure amorphous silicon samples. The total electronic density of states and local density of electronic states (LDOS) at Si atom sites are nearly sample independent, while the LDOS at the H atom sites are dependent on the sample preparation procedure., Comment: 20 pages, 11 figures, Accepted in Physical Review B