1,721,067 research outputs found
Pseudopotential Hartree-fock Study of 17 Iii-v-semiconductors and Iv-iv-semiconductors
No full textThe binding energy (BE), the equilibrium lattice parameter (alpha-0), the bulk modulus (B), and the central-zone transverse-optical phonon frequency (v) of seventeen diamond- and zinc-blende-type-structure semiconductors involving atoms belonging to the second to fifth periods have been evaluated. The periodic ab initio Hartree-Fock linear-combination-of-atomic-orbitals program CRYSTAL has been used. Core pseudopotentials have been adopted in order to limit the calculation to valence electrons. Thirteen atomic orbitals (2s, 6p, 5d) per atom have been used. The quality of the pseudo-potential results is checked by comparison with all-electron calculations performed on six light-atom systems (diamond, silicon, BN, BP, AlP, SiC). The mean errors of the pseudopotential calculations with respect to experimental data are -38%, +1.2%, +6.5%, and +7.8% for BE, alpha-0, B, and v, respectively. A correlation-only density-functional alpha-posteriori correction to the Hartree-Fock total energy is performed, which reduces the absolute BE mean error to 2.6%
Regular Adsorption of Co Molecules On Lif(001)
No full textThe regular adsorption of CO at various concentrations on an infinite three-layer slab of LiF has been studied by using the CRYSTAL ab initio program. The CO molecules are adsorbed vertically above the surface cations by purely electrostatic attraction counterbalanced by the Pauli repulsion. The induced polarization has been analysed through the changes occurring in the molecular orbitals of the adsorbed species. The adsorption via the oxygen atom appears to be preferred, which contradicts IR experimental data. Such contradiction arises from the correlation error in the Hartree-Fock SCF method which is applied in the CRYSTAL program. Consideration of simple correlated clusters enables a semi-quantitative evaluation which additively corrects the above contradictory results
Abinitio Hartree-fock Investigation of the Surface-features of Lih Slabs of Different Thickness
No full textElastic constants and electronic structure of fluorine (CaF2): an ab initio Hartree-Fock study
No full textThe ground state properties of fluorite (CaF2) have been studied using CRYSTAL, an ab initio periodic Hartree-Fock program. Twenty-two and thirteen atomic orbitals (represented as contracted Gaussian-type functions) are used for the calcium and fluorine atoms, respectively. The binding energy (BE), the equilibrium lattice parameter (a), the elastic constants (Cij) and the central zone phonon frequencies nu (IR) and nu (Raman) have been evaluated, and a good agreement obtained with experiment (for instance the error is +2.0, +1.7, -0.5% for BE, a and C11, respectively). The calculated C44 elastic constant reduces from 48 to 44 GPa (experimental value: 37 GPa) when the fluorine atoms are allowed to displace under strain, indicating the importance of inner deformation for shear elasticity. Electron density maps, density of states and band structure plots are reported which confirm the fully ionic nature of fluorite
Non-empirical Pseudopotentials In the Hf-lcao Approach To Crystalline Solids - Comparison To All-electron Results
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Hartree-fock Abinitio Characterization of Ionic-crystal Surfaces With A Slab Model - the (0001) Face of Alpha-al2o3
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B3LYP Simulation of the Full Vibrational Spectrum of 45S5 Bioactive Silicate Glass compared to v-Silica
No full textThe IR spectrum of the Bioglass 45S5 (of composition 46.1 SiO2, 24.4 Na2O, 26.9 CaO, and 2.6 P2O5mol %) has been simulated by means of periodic ab initio B3LYP calculations. The initial glass structureunit cell envisaging 78 atoms was generated through a melt quench process by means of classical moleculardynamics simulations. The molecular mechanics optimized unit cell has then been fully reoptimized(both unit-cell parameters and internal coordinates) at B3LYP level in a periodic approach using Gaussianbasis sets of double- quality by means of the CRYSTAL06 code. Although long-range structural propertiescannot be modeled by using this ab initio approach because of the intrinsic amorphous nature of theglass, the quantum mechanical simulation proved to be extremely effective in predicting and analyzingthe vibrational features of this biomaterial. The effect of modifiers Na and Ca cations on the networkdynamics has been assessed by comparing the 45S5 vibrational spectrum with that of amorphous V-SiO2,both simulated at the B3LYP level. A detailed assignment of each spectral peak to the correspondingvibrational mode of the two glasses has been addressed and the role of cation modifiers on the vibrationalfeatures has been analyzed in greater details. This may be useful to support further IR and Ramaninvestigations of the bulk structure of these materials
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