Linear Combination Of Gaussian Orbitals Research Articles

Predictions of Electronic, Transport, and Structural Properties of Magnesium Sulfide (MgS) in the Rocksalt Structure

We report results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic, transport and bulk properties of rock salt magnesium sulfide (MgS). In the absence of experimental data on these properties, except for the bulk modulus, these results are predictions. Our calculations utilized the Ceperley and Alder local density approximation (LDA) potential and the linear combination of Gaussian orbitals (LCGO). The key difference between our computations and other previous ab-initio DFT ones stems from our use of successively larger basis sets, in consecutive, self-consistent calculations, to attain the ground state of the material. We predicted an indirect (Γ-X) band gap of 3.278 eV for a room temperature lattice constant of 5.200A. We obtained a predicted low temperature indirect (Γ-X) band gap of 3.512 eV, using the equilibrium lattice constant of 5.183A. We found a theoretical value of 79.76 GPa for the bulk modulus; it agrees very well with the experimental finding of 78 ± 3.7 GPa.

Ab-initio computations of electronic and transport properties of wurtzite aluminum nitride (w-AlN)

We report findings from several ab-initio, self-consistent calculations of electronic and transport properties of wurtzite aluminum nitride (w-AlN). Our calculations utilized a local density approximation (LDA) potential and the linear combination of Gaussian orbitals (LCGO). Unlike some other density functional theory (DFT) calculations, we employed the Bagayoko, Zhao, and Williams' method, enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method verifiably leads to the minima of the occupied energies; these minima, the low laying unoccupied energies, and related wave functions provide the most variationally and physically valid density functional theory (DFT) description of the ground states of materials under study. With multiple oxidation states of Al (Al3+ to Al) and the availability of N3− to N, the BZW-EF method required several sets of self-consistent calculations with different ionic species as input. The binding energy for (Al3+& N3−) as input was 1.5 eV larger in magnitude than those for other input choices; the results discussed here are those from the calculation that led to the absolute minima of the occupied energies with this input. Our calculated, direct band gap for w-AlN, at the Γ point, is 6.28 eV, in excellent agreement with the 6.28 eV experimental value at 5K. We discuss the bands, total and partial densities of states, and calculated, effective masses.

Electronic, transport, optical, and structural properties of rocksalt CdO

We report electronic, optical, and structural properties of rocksalt CdO as obtained from first-principle calculations with both the Tran-Blaha modified Becke-Johnson potential using linearized augmented planewave method in WIEN2k and local density approximation (LDA) potential using the LDA Bagayoko-Zhao-Williams-Ekuma-Franklin (BZW-EF) method in implementing the linear combination of Gaussian orbitals. The results are discussed in relation to existing experimental data, particularly to the Burstein–Moss effect.

Electron energy characteristics and one-electron state spectra of surfaces of molybdenum oxide and solid solutions thereof with niobium and technetium oxides with fluoropolymer coatings

The energy spectra of electrons belonging to the surface centers of molybdenum oxide and its solid solutions (interstitial phases) with niobium and technetium oxides have been analyzed. The densities of one-electron states and their atomic orbitals have been calculated for hydroxyl, dehydroxyl, and fluoropolymercoated surfaces. The calculation method is based on the density functional theory using the three-parameter hybrid Becke-Lee-Yang-Parr exchange-correlation potential in the basis 3-21G of linear combinations of Gaussian orbitals.

ON THE POSSIBILITY OF SMALL Mn CLUSTERS HAVING CUBIC SYMMETRY AND THEIR MAGNETIC MOMENTS

Possibility of cubic structures in small manganese ( Mn ) clusters has been studied using the linear combination of Gaussian orbitals (LCGO) method. Local spin density approximation is adopted for the exchange correlation potential. Both high-spin and low-spin states are found for Mn 13 and Mn 19 clusters with the face-centered cubic symmetry; however, no equilibrium lattice constant could be found. For Mn 9 and Mn 15 clusters with the body-centered cubic symmetry, only high-spin states are found. No equilibrium lattice constant could be found for bcc Mn 9; however, a weak possibility of bcc structure in Mn 15 is found. The calculated magnetic moment of Mn 15 is, however, found to be too large compared with the experimental result.

Compton scattering study on the electronic properties of niobium carbide and niobium nitride

Abstract The electronic structure, experimental Compton profile and the chemical bonding mechanism of niobium carbide and niobium nitride are studied on the basis of the band structure calculations, using a self-consistent, all-electron, linear combination of Gaussian orbitals (LCGO) calculation based on density functional theory. Isotropic Compton profiles of niobium carbide have been measured, using a conventional technique based on 59.54 keV gamma-radiation with a solid state detector. The agreement between the experimental and the theoretical momentum density is very good. It is shown that the anisotropies at low momentum are heavily influenced by the particular shape of the Fermi surface. The charge distributions resulting from valence bands in different regions of the unit cell are discussed. The limitations of the rigid band structure model are illustrated and general trends in the chemical bonding are discussed.

Compton scattering studies of the electron momentum distribution in indium phosphide

Abstract The electron momentum anisotropy of indium phosphide has been studied by measuring the directional Compton profiles of indium phosphide single crystals with the use of radiation from an 241 Am gamma source. Three different samples, cut along the [100], [110] and [111] planes, were used. The experimental anisotropy has been compared with the results based on the linear combination of Gaussian orbitals (LCGO) method. The agreement is very good with our theoretical results. It is found that the extrema appearing in the dependences on q of the anisotropies have an intimate connection with the bonding properties of the semiconductor. A self-consistent, all-electron, local density calculation for the partial density of states, total density of states and the charge analysis is also presented here.

Compton profile of cuprous oxide by linear combination of Gaussian orbitals

In this paper we report self-consistent all-electron local-density calculations using the linear combination of Gaussian orbitals method for cuprous oxide. We calculated the directional Compton profile and their anisotropic effect. The spherical average Compton profile has also been calculated and compared with the experimental results. We also calculated the density of states of cuprous oxide.

First-principles LCGO calculation of the magneto-optical properties of nickel and iron.

We report a first principles, self-consistent, all electron, linear combination of Gaussian orbitals (LCGO) calculation of a comprehensive collection of magneto-optical properties of nickel and iron based on density functional theory. Among the many magneto-optical effects, we have studied the equatorial Kerr effect for absorption in the optical as well as soft X-ray region, where it is called X-ray magnetic linear dichroism (X-MLD). In the optical region the effect is of the order of 2\% while in the X-ray region it is of the order of 1\% for the incident angles considered. In addition, the polar Kerr effect, X-ray magnetic circular dichroism (X-MCD) and total X-ray absorption at the L$_{2,3}$ edges, soft X-ray Faraday effect at the L$_{2,3}$ edges have also been calculated. Our results are in good agreement with experiments and other first principles methods that have been used to calculate some of these properties.

Hydrogen bonding: the role of hydrogen delocalization in the AHB bond

The behavior of the A-H...B multiple minimum hydrogen bond can be understood in terms of an analysis similar to the molecular orbital method in molecular electronic quantum theory. Delocalization of hydrogen about two or more wells is described in terms of a linear combination of Gaussian orbitals, each referred to a well minimum. The analysis yields an effective A-B potential mediated by hydrogen. The role of a model three-center interaction in the potential is found to be important and can account for much of the strength of the hydrogen bond

Calculation of momentum distributions in paramagnetic chromium: Compton scattering and positron annihilation

A detailed calculation of the momentum distributions in paramagnetic (BCC) chromium is carried out using the linear combination of Gaussian orbitals (LCGO) method. Using the same electron wavefunctions, theoretical results are obtained for Compton scattering and positron annihilation, and are presented in the form of electron momentum distributions rho (p), reciprocal form factors B(z), two-photon momentum distributions rho 2 gamma (p), one-dimensional angular correlation of positron annihilation radiation, Lock-Crisp-West distributions, etc. These results are analysed in terms of the Fermi surface topology of paramagnetic Cr. In particular, the present results for rho 2 gamma (p) are compared with those reconstructed by Kubota et al (1991) from their two-dimensional angular correlation of positron annihilation radiation measurements. Observed differences are ascribed to the effects of e+-e- many-body correlations and the reconstruction procedure.

Interplanar binding and lattice relaxation in a graphite dilayer.

High-precision, large-basis-set calculations, in the local-density approximation (LDA) (using the all-electron, full-potential, linear combination of Gaussian orbitals, fitting-function technique), of the cohesive properties and electronic states (bare Kohn-Sham energies) of the isolated AB dilayer of graphite are reported. They show that the dilayer interplanar spacing (c axis) differs little from the value for ABABAB... crystalline graphite (0.7% expansion relative to one calculation, 2.5% contraction relative to another, 2% expansion relative to experiment). This result, which differs significantly from a preliminary report of strong c-axis contraction, is related to the weak interplanar binding. The intraplanar lattice spacing (a axis) is virtually identical with the crystalline value for both the graphite dilayer and monolayer. The interplanar binding energy (obtained directly via optimization of the monolayer ground state with the same techniques) is in excellent (perhaps fortuitous) agreement with the experimental value for the crystal, in contrast with crystalline calculations, which are too large (in magnitude) by 40--100 % or more. The dilayer cohesive energy agrees well with the crystalline value from an all-electron calculation. Both exceed the experimental value in magnitude by over 1 eV/atom, a problem already known to arise from inadequacies in the LDA treatment of the multiplet structure of the isolated C atom.The dilayer uniaxial compressibility is much larger than calculated for the crystal, apparently another manifestation of weak interplanar binding. Dilayer Kohn-Sham eigenvalues are largely consistent with those calculated self-consistently for the crystal using the same LDA model. Both differ substantially from the non-self-consistent band structure commonly used to parametrize graphite optical properties of interest in astrophysics. Calculated values of the dilayer work function are larger by about 0.6--0.7 eV than the crystalline experimental results. The dilayer density of states at the Fermi level is predicted to be much smaller than for the crystal, while the occupied bandwidth is in reasonable agreement with crystalline experimental results.

Anisotropy in the electron momentum distribution of silver.

The electron momentum anisotropy of silver has been studied by measuring the directional Compton profiles of silver single crystals with use of the radiation from an annular $^{241}\mathrm{Am}$ source. Three different samples, cut along the (100), (110), and (111) planes, were used. The experimental anisotropy has been compared with the results of two recent calculations. The agreement is very good with the model based on the linear combination of Gaussian orbitals (LCGO) and slightly worse with the augmented-plane-wave model. The directional average has also been measured with use of polycrystalline silver foil; the agreement with the LCGO model is again within the experimental error. A comparison with a previous experiment on polycrystalline silver is also done.

Magnetic Compton profile of nickel

Spin-dependent momentum densities in ferromagnetic nickel have been calculated by using the self-consistent spin-polarized linear combination of Gaussian orbitals (LCGO) method. The resulting magnetic Compton profiles have been compared with the recently measured experimental profiles reported by Timms et al. (1990). The LCGO magnetic profile along (111) direction disagrees with the experimental profile near the origin (pz=0), in that the central dip seen in the experimental profile is not reproduced, a feature shared by APW profile. It is argued that the overestimation of the profiles near the origin is mainly due to electron-electron correlation effects inadequately treated in the local-density approximation.

Calculation of spin-dependent momentum distributions in iron: Compton scattering and positron annihilation

Spin-dependent momentum distributions in ferromagnetic iron are calculated in detail using the linear combination of Gaussian orbitals method. Theoretical results for Compton scattering and positron annihilation are presented in the form of momentum distributions, Compton profiles, one-dimensional angular correlation of annihilation radiation curves, Lock-Crisp-West folded plots and reciprocal form factors B(z), and are compared with experiment wherever possible. It is observed that the present theoretical results show satisfactory agreement with experiment. It is further observed that although the different theories (e.g. augmented plane waves, linear muffin-tin orbitals, present linear combination of Gaussian orbitals) agree among each other about the spin-dependent momentum distributions in ferromagnetic iron, minor differences exist between these theories and experiment. These differences are ascribed to the limitations of the independent-particle approximation, in particular to the effects of non-locality, e--e- and e+-e- many-body correlations, etc.

Mono- and dilayer modifications of lithium lattice parameters

Systematic local density treatment (via the all-electron, full-potential, linear combination of gaussian orbitals fitting function (LCGTO-FF) algorithms) of the cohesive properties of mono- and dilayer Li when combined with recent calculations for bulk Li of similar high quality, yields a prediction of modest intraplanar lattice expansion (dilayer: 2.8%, monolayer: 1.6%) and substantial interplanar contraction (dilayer c/a=1.46 versus calculated bulk c/a=1.64). The differences between these predictions and the limited experimental data for Li overlayers on graphite (which exhibit about 6.1% expansion in nearest-neighbour separation) suggest possible substrate or bonding effects in the experiment. The total dilayer cohesive energy is 79.8% of the bulk cohesive energy while for the monolayer the fraction is 63.1%. The interplanar contribution to the dilayer cohesive energy is -0.29 eV. The dilayer uniaxial (c-axis) compressibility is 2.5 times as large as the calculated value for the HCP Li crystal (the latter value is in quite good agreement with available measurements). Mono- and dilayer energy bands (at the level of bare Kohn-Sham eigenvalues) are basically consistent with those calculated self-consistently for the crystal using the same LDA model. However, calculated work function values are larger than measured crystalline values, by more than 0.6 eV, for both the mono- and dilayers.

Band structure and related properties of molybdenum.

Self-consistent, all-electron, local-density calculations are reported for bcc molybdenum, using the linear combination of Gaussian orbitals (LCGO) method. We obtain the density of states, the Fermi surface, charge form factors, the Compton profile, and the optical conductivity. Results are compared with other calculations and with experiments where these exist. A possible ferromagnetic state is found for an increased lattice constant, developing at about 8.07 a.u.

Theory of anisotropic donor states in quantum-well structures.

The s-, ${p}_{0}$-, and ${p}_{\ifmmode\pm\else\textpm\fi{}}$-like ground and excited states of anisotropic donors in quantum wells are calculated with a linear combination of Gaussian orbitals with different exponents in the z direction and xy plane. The energies and wave functions of the ground and excited states can be obtained simultaneously by this method. In the case of anisotropic donors in the bulk materials and isotropic donors in the quantum wells our results are in agreement with previous theoretical results. In some cases our results are better. Using this method we calculate the variation of the anisotropic donor energies with well widths, anisotropic factors \ensuremath{\gamma}, and impurity positions in the quantum well. The energies of s-like donor states related to the second subband are also calculated. Finally, the donors of the X band edge in Si/Si-Ge quantum wells are discussed.

Studies of the electronic structures of MoSi2

The electronic states of MoSi2 in the body-centred tetragonal CaC2 structure have been studied using the self-consistent linear muffin-tin orbital method. The core states are treated in a fully relativistic way, whereas for the valence states a semirelativistic approximation is utilised. The results show that the majority of Mo d electrons interact with Si p electrons; therefore there exists a strong p-d hybridisation state in MoSi2. The energy band structure is compared with that calculated by the fully relativistic LCGO (linear combination of Gaussian orbitals) method; it was found that the spin-orbit coupling has little effect on the band structure of MoSi2. The total and the projected densities of states are also calculated and compared with the photo-emission data.

A semi-empirical total energy method for elemental clusters and solids: Application to the structural properties of silicon clusters

A semi-empirical pseudopotential method to determine the structural properties of small atomic clusters is presented. The method combines a linear combination of Gaussian orbitals, an atomic-like pseudopotential and a valence force field description to determine the structural energies of small clusters. This procedure is easy to implement and accurately reproduces the structural properties of silicon clusters as determined from more rigorous techniques.