Mulliken Bond Population Research Articles

BAND STRUCTURE, HARDNESS, THERMODYNAMIC AND OPTICAL PROPERTIES OF SUPERCONDUCTING Nb2AsC, Nb2InC AND Mo2GaC

First-principles investigation of the geometry, electronic band structure, Vickers hardness, thermodynamic and optical properties of three superconducting MAX compounds Nb 2 AsC , Nb 2 InC and Mo 2 GaC have been carried out by the plane-wave pseudopotential method based on density functional theory (DFT) implemented in the CASTEP code. The theoretical Vickers hardness has been studied by means of Mulliken bond population analysis and electronic densities of states. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats and thermal expansion coefficient of the three 211 MAX phases are derived from the quasi-harmonic Debye model with phononic effect for the first time. Furthermore, all the optical properties are determined and analyzed for the first time for two different polarization directions. The theoretical findings are compared with relevant experiments (where available) and the various implications are discussed in details.

Effect of lattice defects on the electronic structures and floatability of pyrites

The electronic structures of three types of lattice defects in pyrites (i.e., As-substituted, Co-substituted, and intercrystalline Au pyrites) were calculated using the density functional theory (DFT). In addition, their band structures, density of states, and difference charge density were studied. The effect of the three types of lattice defects on the pyrite floatability was explored. The calculated results showed that the band-gaps of pyrites with Co-substitution and intercrystalline Au decreased significantly, which favors the oxidation of xanthate to dixanthogen and the adsorption of dixanthogen during pyrite flotation. The stability of the pyrites increased in the following order: As-substituted < perfect < Co-substituted < intercrystalline Au. Therefore, As-substituted pyrite is easier to be depressed by intensive oxidization compared to perfect pyrite in a strongly alkaline medium. However, Co-substituted and intercrystalline Au pyrites are more difficult to be depressed compared to perfect pyrite. The analysis of the Mulliken bond population and the electron density difference indicates that the covalence characteristic of the S-Fe bond is larger compared to the S-S bond in perfect pyrite. In addition, the presence of the three types of lattice defects in the pyrite bulk results in an increase in the covalence level of the S-Fe bond and a decrease in the covalence level of the S-S bond, which affect the natural floatability of the pyrites.

Density functional study of electronic,bonding, and vibrational properties of Ca (NH2BH3)2

We present structural, electronic, bonding and vibrational properties of new type hydrogen storage material calcium amidoborane Ca(NH(2)BH(3))(2) by first principles density functional theory using plane wave pseudopotential method. The calculated ground state properties are in good agreement with experiments. The computed Bulk modulus of Ca(NH(2)BH(3))(2) is found to be 28.7 GPa which is slightly higher than that of NH(3)BH(3) indicating that the material is hard over NH(3)BH(3). From the band structure calculations, the compound is found to be a direct band gap insulator with a band gap of 3.27 eV at the Γ point. The calculated band structure shows that the top of the valance band is from the p states of N and the bottom of the conduction band is from d states of Ca. The Mulliken bond populations, Born effective charges and charge density distributions are used to analyze the bonding nature of the compound. It is found that the N-H and B-H bonds are covalent in nature. Further we also compared the phonon density of states and vibrational frequencies of Ca(NH(2)BH(3))(2) with NH(3)BH(3). The study reveals that in both the cases the heavier mass atoms Ca, N, B are involved in the low frequency vibrations whereas the higher frequency vibrations are from H atoms. It is also observed that the vibrational frequencies of B-H bonds are soft in Ca(NH(2)BH(3))(2) when compared to NH(3)BH(3) and thereby concluded that Ca(NH(2)BH(3))(2) is a potential hydrogen storage material for fuel cell applications when compared to NH(3)BH(3).

First-principle calculations of structural, electronic and optical properties of BaHf xTi 1−xO 3

The structural, electronic and optical properties of cubic BaHf x Ti 1− x O 3 ( x = 0, 0.25, 0.5, 0.75) (BHT) are investigated using pseudo-potential plane-wave (PP-PW) density functional theory (DFT) taking the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The calculated structural parameters are consistent with the experimental results. The energy band structures, density of states (DOS), Mulliken charges and bond populations are obtained. The fundamental band gaps of BHT are all indirect and the Hf substitute can induce the band gap of BaTiO 3 widening. The analysis of the structural properties, the Mulliken charges and bond populations shows that the space group of BHT becomes from Pm3 m to P4 mm and spontaneous polarization is caused by the substitution of Ti by Hf. The dielectric function, reflectivity spectrum, absorption coefficient, refractive index, and electron energy-loss spectrum are also calculated and analyzed in details. It is found that dielectric imaginary part of BHT decreases as Hf concentration increases.

First-principle calculations for electronic structure and bonding properties in layered Na2Ti3O7

Abstract First-principles calculations of Na2Ti3O7 have been carried out with density-functional theory (DFT) and ultrasoft pseudopotentials. The electronic structure and bonding properties in layered Na2Ti3O7 have been studied through calculating band structure, density of states, electron density, electron density difference and Mulliken bond populations. The calculated results reveal that Na2Ti3O7 is a semiconductor with an indirect gap and exhibits both ionic and covalent characters. The stability of the (Ti3O7)2− layers is attributed to the covalent bonding of strong interactions between O 2p and Ti 3d orbitals. Furthermore, the O atoms located in the innerlayers interact more strongly with the neighboring Ti atoms than those in the interlayer regions. The ion-exchange property is due to the ionic bonding between the Na+ and (Ti3O7)2− layers, which can stabilize the interlayers of layered Na2Ti3O7 structure.

Ab initio studies of the para- and antiferroelectric structures and local polarized configurations in NH4H2PO4

A study of differently polarized structures relevant to the H-bonded antiferroelectric (AFE) compound NH(4)H(2)PO(4) (ADP) is performed by first-principles calculations in the framework of the density functional theory. The calculated structures for the AFE and paraelectric (PE) phases are found in general good agreement with the available experimental data. We study the energetics and relative stability of different polarized clusters embedded in a PE matrix of ADP. We find that local ferroelectric and AFE clusters are stable and may coexist in the PE phase, which explains the coexistence of both type of microregions determined by electron spin probe measurements above the AFE-PE transition temperature. The dependency with the O-H···O bridge length of the energy barrier heights for proton transfer is studied for coordinated proton displacements along the bridges within clusters of different sizes. This dependency may have implications for the geometric isotopic effects on T(c). We analyze Mulliken orbital and bond populations which confirm the existence of a charge flow within the NH(4)(+) ion, an essential fact for the stabilization of the AFE phase over other possible polarized structures. This charge transfer is correlated with the optimization of the N-H···O bridges and with distortions of the NH(4)(+) group.

Structural, electronic, bonding, and elastic properties of NH3BH3: A density functional study

The structural, electronic, bonding, and elastic properties of the low-temperature orthorhombic phase of NH(3)BH(3) are studied by means of first-principles total energy calculations based on the pseudopotential method. The calculated structural parameters of NH(3)BH(3) are found to be in good agreement with the experimental values. From the band structure calculations, the compound is found to be an indirect bandgap insulator with the bandgap of 5.65 eV (5.90 eV) with LDA(GGA) along the Γ-Z direction. The Mulliken bond population and the charge density distributions are used to analyze the chemical bonding in NH(3)BH(3) . The study reveals that B-H bonds are more covalent than N-H bonds. The elastic constants are predicted for ambient as well as pressures up to 6 GPa, from which theoretical values of all the related mechanical properties such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and anisotropy factors are calculated. It is found that NH(3)BH(3) is mechanically stable at ambient and also external pressures up to 6 GPa. As pressure increases all the calculated elastic moduli of NH(3)BH(3) increase, indicating that the compound becomes more stiffer and hard under pressure. From the ratio of shear modulus to bulk modulus (G/B), we conclude NH(3)BH(3) to be ductile in nature, and the ductility increases with pressure. The present results confirm the experimentally observed less plastic nature of the low-temperature phase of NH(3)BH(3) .

Why Lewis acids accelerate the thermal Curtius rearrangement of benzoyl azide into phenyl isocyanate

The thermal Curtius rearrangement of benzoyl azide in the presence of Lewis acids has been studied by DFT (PBE/TZ2P) method. The complexation of Lewis acids (BF 3, AlCl 3, SbCl 5) with benzoyl azide leads to the formation of 1:1 and 1:2 stable complexes with interaction of catalysts with O and N atoms of carbonyl azide group. The potential energy surfaces of the catalytic rearrangement have been calculated for each complex and the relation between the complexes and the transition states on potential energy surface have been established by IRC calculation. The energy barriers for catalytic reactions are significantly lower in the most cases in comparison with an uncatalyzed reaction. The activation energy is decreasing in the range of Lewis acids AlCl 3 > SbCl 5 > BF 3 and it correlates with the decreasing of Lewis acids strength. The Mulliken bond population analysis has been done for three compounds RCON 3 (R = H, Me, Ph) and for their complexes, and for all corresponding transition states using the B3LYP/6-311G∗ approximation. The interaction of Lewis acids with carbonyl azide group causes the decreasing of N1 N2 bond strength and it helps the thermal Curtius rearrangement to proceed.

Electronic structure and magnetism of Fe 3Al 1−xSi x alloys

Total energy and electronic structure calculations for Fe 3Al 1−xSi x alloys with DO 3-type structure have been performed to understand structural and magnetic properties using plane-wave pseudopotential method. With the increase of Si content, the equilibrium lattice constants reduce linearly and the saturation magnetic moments per unit cell of Fe 3Al 1−xSi x alloys decrease, which are in good agreement with the experimental results. To illustrate the origin of magnetism of Fe 3Al 1−xSi x alloys, spin-polarized density of states and local magnetic moments are investigated. The results show that the difference in local moments of Fe(A,C) and Fe(B) is due to different local environment in DO 3-type structure and the relative affinity of Si and Al for Fe. In addition, to analyze the effect of Si on the magnetism of Fe 3Al, the Mulliken charge and bond population are used to study the charge transfer and the bonding types, respectively. The calculated results give a reasonable explanation of recent experimental findings.

Si-C bond strength in cage-like methylsilsesquioxanes and methyl-bearing metalosilsesquioxanes estimated from DFT calculations

The strength of the Si-C bond in cage-like methylsilsesquioxanes and methyl-bearing coppersilsesquioxanes was estimated from the Mulliken bond populations calculated using the B3LYP density functional method. The estimation was performed using a linear relation between Mulliken bond populations and the calculated Si-C bond strengths in a series of silanes, which is in good agreement with the published data. The introduction of Cu atoms into the silsesquioxane cage leads to a decrease in the Si-C bond strength, which can be a reason for a considerable decrease in the thermal stability of metalorganosilsesquioxanes compared to their siloxane analogs.

First-principles study on the site preference of Dy in B2 NiAl

Abstract The site preference behavior of Dy in B2 NiAl was investigated by a combination of the first-principles and statistical–mechanical Wagner–Schottky method. At T = 0 K, the site occupation of Dy in B2 NiAl is highly stoichiometric-dependent, whereas, Dy showing a consistent preference for the Al sublattices in three types of B2 NiAl at temperatures higher than 300 K. The density of states (DOS) and Mulliken bond population calculations show that the interactions between Al (s, p) electrons and Ni (d) electrons are much stronger in the Dy-doped NiAl than those in pure NiAl, which could be responsible for the improvement of mechanical properties of the Dy-doped NiAl.

Structural stability and magnetism of γ′-Fe 4N and CoFe 3N compounds

Total energy and electronic structure calculations for γ′-Fe 4N and CoFe 3N in the anti-perovskite crystal structure have been performed to understand the structural stability and magnetism using plane-wave pseudopotential method. The virtual crystal approximation (VCA) was used to study order/disorder to examine the effect of the substitution of Co for Fe on the structural stability in CoFe 3N alloy. The results show that CoFe 3N energetically prefers ordered configuration for Co atoms occupying the corner position. Moreover, the substitution of Co for Fe at corner sites does slightly change the lattice constant, whereas it increases bulk modulus and could hardly changes the ductility. To reveal the role of nitrogen in γ′-Fe 4N and CoFe 3N, the Mulliken charge and bond population are used as a measure of charge transfers and an analysis of the covalency of these systems, respectively. It turns out that the further distant atoms from the N atom have the larger magnetic moment for γ′-Fe 4N and CoFe 3N, due to the spin-down electrons transfer from the corner atoms to the face-centered atoms.

Pressure-induced structural and electronic changes inα−AlH3

Pressure-induced structural, electronic, and thermodynamic changes in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{H}}_{3}$ were investigated using synchrotron x-ray powder diffraction and density-functional theory. No first-order structural transitions were observed up to $7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. However, increasing Bragg peak asymmetry with pressure suggests a possible monoclinic distortion at moderate pressures $(1--7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$. The pressure-volume relationship was fit to the Birch-Murnaghan equation of state to give a bulk modulus of approximately $40\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The reduced cell volume at high pressure is accommodated by octahedral tilting and a decrease of the Al-H bond distance. Ab initio calculations of the free energy indicate that hydrogenation becomes favorable at ${\mathrm{H}}_{2}$ pressures above $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Electronic density of states calculations reveal a slight decrease in the band gap with pressure but no evidence of an insulator-to-metal transition predicted by previous high-pressure studies. Calculated Mulliken charges and bond populations suggest a mixed ionic and covalent Al-H bond at $1\phantom{\rule{0.3em}{0ex}}\mathrm{atm}$ with an increase in covalent character with pressure.

Adsorption of single Ag and Cu atoms on regular and defective MgO(0 0 1) substrates: an ab initio study

The DFT slab calculations were performed for Ag and Cu atoms adsorbed on both regular and defective MgO(0 0 1) substrates. Both metal atoms and surface O vacancies ( F s centers) were distributed uniformly with a concentration of one Ag, Cu or F s per 2×2 surface supercell. Surface O 2− ions are energetically more preferable for metal-atom adsorption on a regular substrate as compared to Mg 2+ ions. The nature of the interaction between Ag or Cu adatoms and a defectless MgO substrate is physisorption (despite the difference in the adsorption energies: 0.62 vs. 0.39 eV per Cu and Ag adatom, respectively). Above the F s centers, metal atoms are bounded much stronger when compared with regular O 2− sites (2.4 vs. 2.1 eV per Cu and Ag adatoms, respectively). This is accompanied by a substantial charge transfer towards each adatom (Δ q Cu=0.41 e and Δ q Ag=0.32 e) as well as a formation of partly covalent Me– F s bonds across the interface (Mulliken bond populations p Cu– Fs =0.25 e and p Ag– Fs =0.33 e).

Hartree–Fock study of near-edge gap states in CaF 2 with Na +, Cl − or Sr 2+ impurities

The energy of formation and electronic structure of Na +, Cl − and Sr 2+ impurity centers in CaF 2 have been computed using ab initio Hartree–Fock theory and the supercell approach. The work extends and complements recent results [Solid State Commun. 118 (2001) 569] for Mg 2+ as a substitutional impurity in CaF 2. For Na + substituting for Ca 2+ [S(Na)], charge compensation by an F − vacancy [V(F)] or by a second, interstitial Na + [I(Na)] are both considered. In all cases, geometry optimization is done by relaxing the positions of nearest- and next-nearest-neighbors to minimize the total energy. After correction for electron correlation, the energies of formation increase in the order Mg 2+<Sr 2+<Cl −<S(Na)+I(Na)<S(Na)+V(F), and the Na + results are in agreement with previous Mott–Littleton formation energies. Ion charges, charge density maps and Mulliken bond populations are obtained to show the nature of bonding in the vicinity of the defect. Na + leads to states just above the CaF 2 valence band maximum (VBM), and Na + (and also Mg 2+) produce states just below the conduction band minimum. The results are in qualitative agreement with available optical data for Na + and Mg 2+ impurity effects on CaF 2 near-edge absorption but show that gap states are important in addition to perturbed excitons. A Cl − impurity yields a narrow band of states above the VBM which may significantly affect the deep-ultraviolet transmission of CaF 2. Sr 2+ does not appear to produce states in the CaF 2 gap.

Ab initio study of the effect of doping on stacking faults in GaN

The influence of impurities on the basal plane stacking fault energy in GaN is investigated using density functional theory. It is found that silicon, indium, magnesium and carbon impurities each reduce the stacking fault energy by introducing changes to the bonding properties of the material. These bonding properties are analysed in terms of Mulliken charges and bond populations. It is found that the reduction in stacking fault energy correlates both with a reduction in the average anion charge and with an increase in the overlap population.

Theoretical investigation of bonding in diaspore

The bulk modulus of diaspore, alpha -AlOOH, has been obtained from density functional theory based calculations. The value obtained, B = 148 GPa, is consistent with that previously obtained from elastic constant measurements, but in strong disagreement with values derived from high pressure x-ray diffraction experiments. A Mulliken bond population analysis of the electronic structure implies that the Al-O bonds are significantly covalent, in contrast to findings based on an earlier x-ray diffraction study. On compression, the main change is the increase in the hydrogen-bond strength.

Ab initio study of the isocyanate surface complex over silica and alumina

Ab initio calculations have been performed to study the stability of isocyanate complex (NCO) over silica and alumina surfaces. Mulliken and natural bond population analysis methods have been used in order to analyze charge distributions and the direction of charge transfer processes. The results indicate that the NCO group adsorbs linearly and perpendicularly to both surfaces; namely, on top site over silica and on bridge site over alumina. It was observed that the charge transfer from the oxygen lone pairs of NCO to the NC antibonds produces a weakening of NC link. This phenomenon is more important over alumina, yielding to an easier NCO decomposition over the surface of this oxide.

Theoretical study of water coverage on MgO surfaces

Ab initio and semiempirical calculations have been performed on an (MgO)16 cluster model in order to study the effects of water coverage on pure MgO (100) surfaces. The geometries of various adsorbed water molecules have been optimized and the binding energies, charge transfer, and preferential sites of interaction analyzed. We have used Mulliken and natural bond population analysis methods in order to analyze charge distributions and the direction of charge transfer processes. We have also investigated the effects of low and high coverage on energy gaps, density of states, self-consistent field (SCF) orbital energies, and stretching frequencies. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 153–165, 1999

Force-constant model of a fullerene based on a first-principles method and bond-population analysis: Applications to C60 and C70.

We propose a method based on first-principles molecular-orbital (MO) calculations and a force-constant model with Mulliken's bond-population analysis to calculate vibrational properties of fullerenes. In this force-constant model, the parameters are determined by first-principles MO calculations based on the nonlocal-density-functional formalism. This method may be successfully applied to any higher fullerenes, because the number of force constants is extremely reduced by using the bond-population analysis. The calculated vibrational frequencies for ${\mathrm{C}}_{60}$ agree well with experimental ones. Also, we calculate the vibrational frequencies of ${\mathrm{C}}_{70}$ and reveal that three types of covalent bonds exist, double bonds, single bonds, and 1.5 bonds. We give a reasonable explanation to this remarkable feature of ${\mathrm{C}}_{70}$.