Semiempirical Intermediate Neglect Of Differential Overlap Research Articles

Semiempirical LUC‐INDO calculations on the effect of pressure on the electronic structure of diamond

AbstractA self‐consistent LUC (large unit cell) formalism on the basis of semiempirical INDO (intermediate neglect of differential overlap) Hamiltonians has been used to study the electronic properties of diamond and to investigate the pressure dependence of these properties. The calculated properties are in good agreement with the experiments except the conduction‐band width. The increase of pressure on diamond is predicted to cause the following effects; an increase of the valence and conduction‐band widths with a decrease of the direct bandgap, an increase of the electronic occupation probability for the p‐orbital with a decrease of this probability for the s‐orbital, and a decrease of the X‐ray scattering factor. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computer Modeling of Point Defects in Perovskite Crystals

The results of calculations for mixed perovskite KNb x Ta 1 m x O 3 (KTN) solid solutions are presented and discussed. The semiempirical method of the intermediate neglect of differential overlap (INDO) based on the Hartree-Fock formalism predicts magnitudes of the Nb off-center displacements which are in agreement with EXAFS data. Results are compared with ab initio FP-LMTO first-principles calculations. The latter formalism and the semiempirical INDO methods are combined for the study of hole and electron polarons in perovskites. We calculated the defect migration energies using INDO method and classical shell model (SM) for the cubic phase of the KNbO 3 . The migration energies for the O vacancy obtained by these methods are reasonably close and agree with the only experimental estimate available of 1 eV.

Electronic and spatial structures of the hydroxyl ion in LiF, NaF, and KF crystals

The electronic and spatial structures of the impurity hydroxyl ion in LiF, NaF, and KF crystals are studied using a molecular cluster model by the Hartree-Fock-Roothaan method in the semiempirical INDO (intermediate neglect of differential overlap) approximation. The equilibrium geometry of the impurity ion is determined. The hydroxyl ion is oriented in all the three matrices along the 〈110〉-axis, which is caused by the formation of the selected chemical bond between the hydrogen ion and the nearest lattice fluorine ion located on the same axis. The optical absorption energies are calculated.

Semi-empirical indo and shell-model calculations for perovskites

Structural, phonon and some elastic and dielectric properties have been calculated for various paraelectric or ferroelectric phases of the perovskites KNbO3 and SrTiO3, using either the semi-empirical INDO (Intermediate Neglect of Differential Overlap) method or a temperature-dependent shell model. The INDO method was used to calculate the energy changes resulting from [100], [110] or [111] displacements of Nb atoms in the cubic perovskite cell of KNbO3, at 0K. The conventional shell model gives a good account of the elastic, dielectric and phonon properties of the cubic phase of strontium titanate at room temperature, but difficulties remain in modelling the permittivity and elastic properties of KNbO3.

Spectral Properties of Some Naphthalene Derivatives of Bicarboxylic Acid; Comparison of Experimental Results with Theoretical Predictions

Abstract The basic electronic quantities of some dichroic dyes, naphthalene derivatives of bicarboxylic acid, have been evaluated theoretically using a semi-empirical INDO (intermediate neglect of differential overlap) Cl (configuration interaction) approach. The theoretical data have been com­ pared with the experimental results obtained from the absorption and fluorescence measurements of the dyes dissolved in toluene and oriented in liquid crystal 8CB. Additionally, the order parame­ ters of the dyes in smectic A and nematic phases of 8CB have been determined.

Calculation of UV/VIS absorption spectra of liquid crystals and dye molecules An INDO MO approach

Abstract The electronic transitions in the UV/VIS range of liquid crystals and stilbene dyes, which are of commercial and applicational interest, have been studied theoretically by a semi-empirical INDO (intermediate neglect of differential overlap) CI (configuration interaction) approach. Experimentally obtained polarized UV/VIS spectra of the systems investigated can be reproduced with reliable accuracy by the theoretical model. Measured and calculated π-π∗ transitions differ frequently by less than 30 nm. The main absorption bands are strongly polarized parallel to the principal molecular axis.

Conformational analysis of allyl halides from the calculation of indirect spin-spin coupling

A theoretical study on the conformation of allyl halides from the calculation of nuclear spin-spin coupling constants by adopting the finite perturbation theory (FPT), is carried out in terms of the self-consistent, semi-empirical INDO (intermediate neglect of differential overlap) approximation of molecular orbital theory. Results of the calculations performed using ‘s’ and ‘p’ valence orbitals alone (‘sp’ basis) at INDO level approximation seem to replicate the experimental trend quite satisfactorily. Despite the overall agreement of the theoretical values with the experimental ones, the uncertainties in the INDO parametrization scheme lead to overestimation of certain coupling constants. The calculations also show that the orientation of the coupled protons with respect to the substituent halogen atom is an important factor to be considered.

The band structure of one-dimensional (tetrazaporphyrinato)cobalt(II). A semi-empirical self-consistent field crystal orbital analysis

The band structure of (tetraporphyrinato)cobalt(II) ( 2) has been investigated by means of a semi-empirical INDO (intermediate neglect of differential overlap) crystal orbital approach. The simplest stoichiometric unit of the title compound contains an uneven number of electrons. We have considered metallic and insulating (Mott) electronic distributions in the “half-filled” band. The Hartree-Fock energies for two spatial symmetries have been determined: (i) singly occupied Co 3d z 2 band (a 1g) and (ii) singly occupied ligand-centered (a 1u) linear combination. The mean-field energies of insulating “Mott” states have been calculated by means of a grand canonical (GC) averaging procedure. The electronic ground state is of spatial A 1g symmetry (“half-filled” Co 3d z 2 dispersion) and corresponds to an insulating band occupation. The calculated width of the a 1g dispersion amounts to 0.13 eV. It is argued that the band structure results of 2 are in qualitative agreement with available experimental results derived for (phthalocyaninato(cobalt(II).

Charge distribution in Ni6H, Ni14H and Ni38H clusters

Charge distributions in NinH clusters with n=6, 14 and 38 are calculated by a semi-empirical INDO (intermediate neglect of differential overlap) Hartree-Fock SCF (self-consistent field) method. The computations in this formalism allow also for the simulation of small H concentrations (e.g. n=38) in the Ni lattice. The magnitude of the charge transfer is studied as a function of n (i.e. size of the Ni cluster) and the lattice parameter. The influence of intra-atomic correlations on the magnitude of the charge transfer is estimated by an 'atom in molecule' type of approach. The semi-empirical INDO results are compared with local density functional calculations using the LMTO formalism and calculations of a proton in jellium. It is concluded that the proton in Ni can be interpreted as an overscreened particle and that the electronic redistribution caused by H in Ni is localised within the first shell surrounding the proton site.

The band structure of the one-dimensional tetrathiotetracene system

Abstract The band structure of the one-dimensional (1D) tetrathiotetracene (TTT) system 1 has been studied in the framework of a semiempirical INDO (intermediate neglect of differential overlap) crystal orbital approach based on the tight-binding approximation. The unoxidized TTT polymer is an insulator with a forbidden band gap of 4.42 eV. The width of the valence band amounts to 1.87 eV and is in remarkable agreement with experimentally deduced values. The “correlation strength” in 1 is analyzed by means of the convenient Hubbard parameter U/4t (U: on-site repulsion, 4t: band width); the calculated U/4t ratio is both far from the “atomic limit” U/4t→∞ and the tight-bonding regime U/4t→0. The computational results are compared with available experimental data.

Symmetry violations in partially oxidized one-dimensional (1D) transition metal polymers. The Ni(CN)2−4 system

The band structure of tetracyanonickelate (II), Ni(CN)2−4 1 has been studied in the unoxidized state and in a partially oxidized form with one electron removed per two Ni(CN)2−4 sites. Computational tool for this investigation is a semiempirical INDO (intermediate neglect of differential overlap) crystal orbital (CO) model based on the tight-binding formalism and combined with the grand canonical (GC) averaging procedure. This approximation allows for the formation of average values over certain manifolds of one- (or many-electron) states and thus makes possible the investigation of violations of the spatial symmetry separated from metal–insulator transitions and spin-dependent phenomena. The oxidation process in 1 is metal centered (3dz2 states) and leads to a broken symmetry CDW (charge density wave) solution with trapped valences at the Ni sites within the allowed variational space in the Hartree–Fock (HF) CO formalism. The charge separation between the transition metal centers ΔqNi amounts to ∼0.87e. The band structure shows some analogies to the dispersion curves encountered in semiconductor superlattices. The physical origin for this valence trapping is analyzed and compared with violations of spatial symmetries in finite transition metal complexes, clusters as well as simple solids. The title compound belongs to a class of organometallic polymers in the 3d series where the covalent metal–metal interaction (kinetic energy operator) is negligibly small in comparison to the strongly repulsive two-electron part. The symmetry adapted HF state of the partially oxidized backbone with averaged populations at the Ni sites is thus unstable against a condensation into an arrangement with alternating oxidation states (trapped valences).

Theoretical Studies of Solvent Effects on Nuclear Spin—Spin Coupling Constants. II A Cubic Closest Packed Cluster Model

A theoretical study of the solvent dependence of nuclear spin—spin coupling constants is presented in terms of a model which assumes a cubic closest packed cluster in which six solvent molecules are arranged around the solute molecule. This model is described theoretically by means of a group function modification of self-consistent perturbation theory. Calculations for individual molecular groups were performed by the semiempirical INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. Intermolecular integrals associated with solute and solvent molecules were evaluated theoretically. This model provides some improvement over the previous theoretical study in this series, which was based on the reaction field model. The improvement can be attributed to the inclusion of electrostatic effects arising from the real charge distributions of the solvent molecules, especially those which are perpendicular to the solute dipole moment axis. A most important result is the good agreement found for the solvent dependence of the geminal H–H coupling constant in formaldehyde, as the reaction field results for this case were of the wrong sign. The calculated solvent dependence of the coupling constants of 1, 1-difluoroethylene in solvents of almost identical dielectric constant exhibits a greater range of values than either the experimental values or the reaction field results. On this basis, it is concluded that the cluster model is too specific and that improvement in the theoretical description of this phenomenon will incorporate rotational effects associated with the solvent molecules.

Theoretical Studies of Solvent Effects on Nuclear Spin–Spin Coupling Constants. I. The Reaction Field Model

A theoretical study of the solvent dependence of nuclear spin–spin coupling constants is presented in terms of Onsager's reaction field model and self-consistent perturbation theory in the semiempirical INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. A possible mechanism for reaction field induced changes in coupling constants is discussed in terms of the sum over virtual orbital method, corresponding to the uncoupled Hartree–Fock approximation. Although the latter method does not give satisfactory values for coupling constants, the reaction field induced shifts are reasonable. Finite perturbation theory, which corresponds to the coupled Hartree–Fock approximation, gives much better coupling constant results. Calculated reaction field induced shifts for a variety of H–H and H–F coupling constants are compared with the experimental results in a variety of solvents. Although the calculated results are linear functions for very large values of the reaction field, best agreement with experiment is obtained in solvents of low dielectric constant. The importance of solute dipole orientation in a series of fluoroethylenes is investigated.