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The nature of bonding in the cubic cuprous oxide is studied by means of the theoretical tools, namely, the electron localization function and Compton profiles. The isotropic Compton profiles together with the anisotropies in the directional Compton profiles are presented. Taking free-atom Compton profiles, the charge-transfer model is also applied. The first-principles calculations based on the GGA are performed, and the self-interaction correction is incorporated, adopting the GGA+U approach. Both types of calculations are performed deploying the linearized augmented plane-wave (LAPW) method. The effect of selfinteraction correction on the electron localization function, Compton profiles, and anisotropies is discussed. The electron localization function reveals ionic behavior in the ( 110) plane and covalent nature in the Cu−O bond intersecting plane. The GGA+U exhibits more covalent nature. The two LAPW calculations of the Compton profiles show better agreement with the available experimental data than the free-atom profiles. Among all of the calculations undertaken, the GGA+U shows the best agreement with the experiment. The GGA+U calculation shows more anisotropic behavior in directional Compton profiles.

We present the first-principles calculations under the framework of density functional theory to explore the Fermi surface and electronic properties of covellite mineral. The correlation effects are considered applying the +U correction in the density functional theory. Lattice parameters are determined and the possibility of pressure induced phase transitions to the hypothetical B1 and B3 crystals is examined. All calculations show impending B18 → B1 and B3 → B1 phase transitions. Using generalized gradient approximation these are found to occur at 7.4 and 6.48 GPa, respectively. Electronic bands structures of the three crystals highlight metallic properties. Two copper atoms situated at distinct locations in covellite exhibit a distinct role. The Fermi surfaces of all phases are presented. The calculations of B18 map out corrugated cylindrical Fermi surface signifying inter-layer interaction mediated by the S(II)-S(II) bond. The +U correction shows anisotropy in the Fermi surface noted in experiment. It also indicates stronger inter-layer interaction. Applying Debye-Slater and the Debye-Grüneisen models the thermal expansion coefficient, heat capacity and entropy are found and their temperature dependence is discussed.

Density functional theory at the level of LDA, GGA, LDA + U, GGA + U and hybrid functionals is applied to investigate structural and electronic properties of three RuO2 crystals. The rutile structure, and the pyrite and flourite modifications of RuO2 are undertaken. The structural properties, enthalpy-pressure curves, electronic states, and Fermi surfaces are presented. The enthalpy-pressure curves show that pressure causes the rutile-RuO2 to transform into pyrite and flourite phases. The pyrtie phase transforms in the fluorite phase. All calculations point out pressure induced rutile → pyrite phase transition in confirmation with the experimental studies. The pyrite → fluorite transition is pointed out by current calculations. The rutile and pyrite crystals are metals while hypothetical fluorite is a semiconductor. All calculations show s that the fluorite has an indirect bandgap in the 0.57–2.96 eV range. The Fermi surface of metallic rutile structure using GGA + U shows improvement over GGA on comparison with the measurement. The GGA + U calculations suggest that rutile → fluorite and pyrite → fluorite metal-insulator transitions are accompanied by orbital ordering.

The thermoelectric properties of semiconducting compound ZnSb are studied using crystalline orbitals program based on the periodic linear combination of atomic orbitals method. The calculations are done under the framework of density functional theory. We calculate the electronic band structure and the density of states. The k-space eigenvalues are coupled with Boltzmann transport equations to calculate transport coefficients such as the Seebeck coefficient, power factor and electronic thermal conductivity under the constant relaxation time and the rigid band approximations. Effect of the scissor correction on the transport coefficients is examined. We have found that ZnSb behaves as n-type thermoelectric. A comparison with available measurements is done and a good agreement is found. The thermoelectric performance is compared with other materials by means of the electronic fitness function which suggests ZnSb to be a good thermoelectric material.