Non-empirical Results Research Articles

Supercomputer modelling of an electronic structure for KCl nanocrystal with edge dislocation with the use of semiempirical and nonempirical models

In the present work we used semiempirical and non-empirical models for electronic states of KCl nanocrystal containing edge dislocation for comparison of the obtained results. Electronic levels and local densities of states were calculated. As a result we found a reasonable qualitative correlation of semiempirical and non-empirical results. Using the results of computer modelling we discuss the problem of localization of electronic states near the line of edge dislocation.

On an inconsistency between experimental and non-empirical data on the lifetimes of electronic–vibrational–rotational states of the H2, HD, and D2 molecules

A comparative analysis is carried out for all published to date experimental and non-empirical data on radiative lifetime of electronic–vibrational–rotational (EVR) levels for the three most common isotopologues of the hydrogen molecule. It is found that 792 available experimental values are extremely fragmentary. The majority of EVR levels have been studied only in one work. The available data give no clear notion about dependences of the radiative lifetime on the vibrational and rotational quantum numbers and complicates comparison of the results obtained by means of different methods and/or in different works. Comparison with non-empirical results is hampered by the absence of computational results for 24, 30, and 90% experimentally studied EVR levels of H2, D2, and HD, respectively. It is shown that, for a significant number EVR levels (46, 68, and 80%, respectively, for H2, D2, and HD), which have been studied both experimentally and theoretically, there is a direct contradiction between experimental and computational results on the radiative lifetime. More accurate and independent experimental measurements of the radiative lifetimes and more accurate non-empirical calculations with the use of nonadiabatic models are needed to resolve the revealed contradictions.

Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6:U4+ crystals. The 5f17s1 manifold

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pedrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U(4+)-doped Cs(2)GeF(6). Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF(6))(2-) and (UF(6)Cs(8))(6+) clusters embedded in Cs(2)GeF(6), in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF(6) volume and their U-F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f(1)7s(1) configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals.

Computational study of the addition of molecular oxygen to benzene

The first step of the attack of the oxygen molecule, in its ground state, on the phenyl ring, in the gas phase, has been studied using SCF ab initio, BECKE3LYP DFT and AM1 and PM3 semiempirical calculations. This attack follows several stages: fixation of the oxygen molecule in the para position, then its dissociation followed by a two-step migration of hydrogen atoms, leading finally to hydroquinone. The potential curves obtained by DFT and SCF calculations are similar; the same stationary points are observed in both methods. However, the SCF energy barriers are overestimated by a factor greater than 30%. The semiempirical computations reproduce approximately some features of the non-empirical results.

Critical Aspects of Measurement of Circular and Linear Dichroism: A Device for Absolute Calibration

The polarization effect of one or several tilted, parallel air-spaced fused silica plates is calculated. It is generally necessary to include higher order reflections. A device consisting of 1–4 plates tilted 10–45° from normal incidence is proposed for the calibration of differential linear dichroism (LD) in the measurement ranges 10−3 to 10−1 decadic absorbance units and the wavelength region 200–700 nm. The experimental LD measured in these ranges on commercial circular dichrometers, converted to LD mode according to Norden and Davidsson, was in excellent agreement with the calculated, nonempirical results. Since the response function of the instrument in this conversion mode is the same for LD and CD, the method also provides a means for absolute calibration of CD. The results can also be applied to polarized scattering and fluorescence. (For previous papers on this theme, see Refs. 6, 9 and 12.)

Hartree—Fock—Slater functions as basis for one-electron perturbation calculations for molecules: I. Calculation of ground state electric dipole polarizabilities

The analysis of the decoupling of Hartree—Fock—Slater SCF perturbation equations for an external field is undertaken. The points of departure from the corresponding Hartree—Fock perturbation equations are stressed. Both formal and numerical results suggest that the fully uncoupled Hartree—Fock—Slater expression is a less drastic approximation than the same Hartree—Fock one. The uncoupled expression for the ground state electric dipole polarizability is calculated for CO, N 2, ethylene, acethylene and trans-butadiene in the dipole length—dipole length, dipole velocity—dipole length and dipole velocity—dipole velocity alternative formulations with an ab initio Hartree—Fock—Slater SCF basis set. The results compare well with other non-empirical results and the dipole velocity-dipole length results are in remarkably good agreement with experiments.

Second-row molecular orbital calculations: II. Semi-empirical calculations of dipole moments

The ability of four semi-empirical methods to predict dipole moments of molecules containing atoms in the second row of the periodic table is investigated. None of the methods is capable of consistently reproducing either magnitudes or qualitative trends; however, the CNDO method of Santry gives the best agreement overall. The original CNDO method of Santry and Segal emphasizes the importance of d orbitals to a greater extent than does the Santry method. Comparisons are presented with non-empirical results when possible.

Simulation of SCF perturbation theory by a simple model potential method. Polarisabilities of divalent atomic species

A simple model potential approach is described for the calculation of static and dynamic polar-isabilities of atomic species containing an s2 valence pair. Results differ considerably depending on whether coupling between the perturbed orbitals is neglected, values derived with the inclusion of intra-shell self-consistency being in good agreement with polarisabilities found by all-electron Coupled Hartree–Fock methods. Polarisabilities are given for a number of systems for which accurate, non-empirical results are not yet available.

Approximate variation-perturbation calculations of second-order molecular properties

The variation-perturbation method is applied to the calculation of the magnetic susceptibility of diatomic molecules within the basis of the CNDO/2 SCF wave functions. All the many-centre integrals which appear in the treatment were simplified according to the ZDO approximation. The calculated diamagnetic contribution is in good agreement with non-empirical results. It agrees also with available experimental data. Although the paramagnetic part of the susceptibility tensor was calculated with a simple two-term form of the variational correction, the results are comparable to those obtained in more advanced ab initio treatments. Also the computed total magnetic susceptibility of various diatomic molecules is in substantial agreement with non-empirical results. A comparison is made with the results of other semi-empirical calculations based on the CNDO/2 SCF wave functions. It follows that the results of the present method are remarkably better.

Approximate variation-perturbation calculations of the second-order molecular properties

The variation-perturbation technique is applied to the calculation of the electric polarizability tensor of diatomic molecules within the CNDO/2 approximation. The results obtained in this paper are similar to those found in the non-empirical variation-perturbation approach within the minimal STO basis set. They are also compared with other CNDO/2 calculations and with existing experimental data. It follows from this comparison that the CNDO/2 variation-perturbation method does not allow for the calculation of the polarizability anisotropies. On the other hand, the rotational average of the polarizability tensor can be predicted in agreement with both the experimental data and nonempirical results.