Albert Lichanot

The phase transition in alkaline-earth oxides: a comparison of ab initio Hartree-Fock and density functional calculations

Structural properties of the (NaCl-type) and (CsCl-type) phases of alkaline-earth oxides and their phase transition have been investigated with the periodic ab initio linear combination of atomic orbitals method implemented in the CRYSTAL program. The geometries have been optimized and the bulk modulus evaluated. The calculations have been done at the Hartree-Fock (HF) and density functional theory (DFT) levels. In this last case, the exchange-correlation potential correcting the electronic density uses either one local or three non-local models. The comparison of the different approaches allows us to identify a trend, in order to obtain results in better agreement with experiment.

Electronic structure of α-MnS (alabandite): an ab initio study

The structural, electronic and magnetic properties of α-MnS are investigated using an ab initio periodic linear combination of atomic orbitals self-consistent field approach. The geometry is optimized and the electronic structure discussed in terms of charge and spin density maps, band structure and density of states. The relative stability of the antiferromagnetic AF2 phase with respect to the ferromagnetic one is discussed as a function of the metal-metal distance.

Electronic, magnetic structures and neutron diffraction in B1 and B3 phases of MnS: a density functional approach

Abstract The linear combination of atomic orbitals (LCAO) method implemented in the C rystal program for the study of periodic systems has been used to obtain wave functions, charge and spin densities of the most stable antiferromagnetic orderings of the MnS B 1 and B 3 polymorphs. The calculations have been led to the density functional (DF) level of theory by solving the Kohn–Sham equations self-consistently. One local and one gradient exchange-correlation (XC) potentials have been studied for comparison with the Hartree–Fock (HF) results. Electron charge and spin densities have been calculated and the neutron diffraction of the two polymorphic forms of MnS deduced. Comparison of our results with the experimental data is discussed.

A comparative study of the electronic structure of α-MnS (alabandite) calculated at the Hartree-Fock and Density Functional levels of theory

Abstract Geometry and electronic structure of α -MnS in its ferromagnetic state are investigated using the periodic linear combination of atomic orbitals self consistent field method implemented in the CRYSTAL95 code. Several approaches combining different exchange and correlation potentials parametrized according to a local or non local correction of the electronic density are examined and compared. For each approach, the geometry is optimized and the electronic structure is discussed in terms of charge and spin densities, band structures and density of states. With respect to the available experimental data, the DFT approaches seem to lead to more satisfactory results.

Structural, electronic, and vibrational properties of solid Sr(OH)2, calculated with different Hamiltonians

The structural equilibrium parameters, the energetic of formation and hydration processes, and the O–H vibrational frequencies of crystalline Sr(OH)2 have been investigated at the ab initio level using the periodic CRYSTAL package. Both Hartree–Fock (HF) and density functional theory (DFT) Hamiltonians have been used, the latter in its local density (LV), gradient-corrected (PP), and hybrid (B3LYP) versions. The computed Sr(OH)2 structural parameters are in reasonable agreement with experiment, the largest deviation being for the a cell parameter, which is overestimated by all the adopted methods. With respect to experiment, DFT Hamiltonians give errors of the order of 13% for the formation energies, whereas errors for the heats of hydration from the corresponding oxide are as large as 27% for the LV functional. Two families of OH groups occur in the structure, in which one acts as a weak hydrogen bond donor. The fundamental ω01(OH) stretching frequency has been computed for the two OH groups, and their d...

Ab initio self-consistent calculations of the Compton profiles and polarizabilities of diamond and cubic boron nitride

Compton profiles, polarizabilities and related functions of diamond and cubic boron nitride have been investigated within the Hartree-Fock approximation and the density functional theory, calculated within the local density approximation and generalized gradient approximation, but without any explicit correlation correction for the Compton profiles. The correlation part already included in the standard uncorrected density functional theory is deduced from the comparison of the two types of calculation. The Compton profile and reciprocal-form-factor anisotropies, polarizability, dielectric constant and energy loss function of the two compounds are compared at the same level of accuracy. These properties are very close in spite of the rather different chemical bonds due to the charge transfer occurring in cubic boron nitride and gaps.

Compton profiles of the isoelectronic BeO and BN molecules: an ab initio study

Abstract Average and directional Compton profiles have been calculated for two linear twelve-electron systems, beryllium oxide and boron nitride. Calculations have been done both at the self-consistent field and configuration interaction levels in order to quantify the electron correlation effect which depends largely on the nature of the chemical bond. Various interatomic distances and basis sets have been considered to compare later the results obtained on these molecular systems to other calculated ones in the corresponding periodic structures.

Density functional LCAO calculations of periodic systems. Effect of an ‘a posteriori’ correction of the Hartree-Fock energy on the physical properties of ionic sulfur compounds

Abstract Density functional theory is taken into account to correct ‘a posteriori’ the total energy calculated with ab initio Hartree-Fock theory. Two models are adopted: the first one is the local density approximation parametrized by Ceperley and Alder while the second one is non-local and corresponds to the formulation proposed by Perdew. The changes in physical properties of ionic sulfide compounds (equilibrium geometry, formation energy, elastic constants, phase transition B1 → B3 in MgS and surface relaxation) associated with the electronic correlation are evaluated.

Compared electron charge densities for the series of solid phosphide compounds; an ab initio study

The wave functions of boron, aluminium and gallium phosphides have been calculated at both the Hartree - Fock and local density approximation levels using the ab initio linear combination of atomic orbitals method implemented in the program CRYSTAL. The Mulliken populations, bandstructure, electron charge densities, and structure factors are evaluated and compared. The chemical bonds are mainly covalent, as revealed by the difference electron charge-density maps obtained by subtracting from the total densities the superposition of the spherical atomic densities. A charge-density accumulation between the nearest neighbours slightly shifted towards the phosphorus appears in the AlP and GaP compounds, whereas an opposite charge transfer from phosphorus to boron is observed in boron phosphide from the Mulliken population analysis.

Reactions amines aromatiques-resines epoxydes: enthalpie et capacites calorifiques

Abstract The overall heat of reaction between aromatic amines and epoxy resins was measured for model and polymer systems. Heat capacities of model, polymer and non stoichiometric systems have also been determined as a function temperature. All data are in agreement with the idea of a complete reaction and, in some cases, secondary reactions.