Full potential linearized augmented plane waves method with the generalized gradient approximation for the exchange-correlation potential was applied to calculate the optical properties of (7, 7) single walled carbon nanotubes. The both x and z directions of the incident photons were applied to estimate optical gaps, dielectric function, electron energy loss spectroscopies, optical conductivity, optical extinction, optical refractive index and optical absorption coefficient. The results predict that dielectric function, ε (ω), is anisotropic since it has higher peaks along z-direction than x-direction. The static optical refractive constant were calculated about 1.4 (z-direction) and 1.1 (x- direction). Moreover, the electron energy loss spectroscopy showed a sharp π electron plasmon peaks at about 6 eV and 5 eV for z and x-directions respectively. The calculated reflection spectra show that directions perpendicular to the tube axis have further optical reflection. Moreover, z-direction indicates higher peaks at absorption spectra in low range energies. Totally, increasing the diameter of armchair carbon nanotubes cause the optical band gap, static optical refractive constant and optical reflectivity to decrease. On the other hand, increasing the diameter cause the optical absorption and the optical conductivity to increase. Moreover, the sharp peaks being illustrated at optical spectrum are related to the 1D structure of CNTs which confirm the accuracy of the calculations.

Structural, elastic, optical, thermodynamical, and electronic properties of yttrium oxide compound in cubic phase have been studied using the full-potential augmented plane waves (FP-LAPW) within density functional theory (DFT) framework. Four different approximations were used for exchange-correlation potentials terms, comprised Perdew-Burke-Ernzerhof generalized parameterization of gradient approximation (GGA-PBE), Wu-Cohen (WC-GGA), localdensity approximation (LDA), and new approximation modified Becke and Johnson (mBJ-GGA). The structural properties such as equilibrium lattice parameter, bulk modulus and its pressure derivative have been obtained using optimization method. Moreover, Elastic constants, Young's modulus, shear modulus, Poisson's ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature, and Gr€ uneisen parameters have been calculated. Obtained structural, elastic and other parameters are consistent with experimental data. Moreover pressure dependence of the elastic moduli was studied. From electronic calculations, it has been found that the band gap was 5.7 eV at Г point in the Brillouin zone using mBJ-GGA approximation. Optical properties, such as the dielectric function, refractive index, extinction index, and optical band gap, were calculated for radiation up to 14 eV. In addition, the unique type of bonding in Y 2 O 3 was discussed by three method including effective charge, B/G ratio, and charge density distribution.

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Electronic, optical, elastic, properties of Copper nitride (Cu 3 N) in cubic anti-ReO 3 phase have been studied using the full-potential augmented plane waves (FP-LAPW) within density functional theory (DFT) framework. Generalized gradient approximation (GGA), local density approximation (LDA), Perdew-Burke-Ernzerhof generalized parameterization of gradient approximation (GGA-PBE), and new modified Becke and Johnson GGA (MBJ-GGA) have been used for exchange-correlation potentials. The structural properties such as equilibrium lattice parameter, bulk modulus and its pressure derivative have been obtained and optimized. The Hubbard potential has been enhanced to improve bandgap energy. Optical properties, such as the dielectric function, refractive index, extinction index, and optical band gap, were calculated for radiation up to 14 eV. The chemical bonding in Cu 3 N was discussed by three method electronegativity concept, B/G ratio, and charge density distribution. Moreover, Elastic constants, Young's modulus, shear modulus, Poisson's ratio, sound velocities for longitudinal and shear waves, Debye average velocity and Debye temperature have been calculated. The estimated structural, elastic and other parameters are in good agreement with experimental data. The calculation exhibits that Cu 3 N is a direct semiconductor (0.7-1.12 eV) with ductile and ionic identity.

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