A perspective on the localizability of Hartree–Fock orbitals

A common perception about molecular systems with a nonlocal electronic structure (as manifested by a nonlocal Hartree–Fock (HF) density matrix), such as conjugated π-systems, is that they can only be described in terms of nonlocal molecular orbitals. This view is mostly founded on chemical intuition, and further, this view is strengthened by traditional approaches for obtaining local occupied and virtual orbital spaces, such as the occupied Pipek–Mezey orbitals, and projected atomic orbitals. In this article, we discuss the limitations for localizability of HF orbitals in terms of restrictions posed by the delocalized character of the underlying density matrix for the molecular system and by the orthogonality constraint on the molecular orbitals. We show that the locality of the orbitals, in terms of nonvanishing charge distributions of orbitals centered far apart, is much more strongly affected by the orthogonality constraint than by the physical requirement that the occupied orbitals must represent the electron density. Thus, the freedom of carrying out unitary transformations among the orbitals provides the flexibility to obtain highly local occupied and virtual molecular orbitals, even for molecular systems with a nonlocal density matrix, provided that a proper localization function is used. As an additional consideration, we clear up the common misconception that projected atomic orbitals in general are more local than localized orthogonal virtual orbitals.     

Conduction and valence band density of states of SnS2: Theory and experiment

The band structure of SnS₂ has been investigated over a wide energy range by pseudopotential band structure calculations and synchrotron radiation photoemission spectroscopy techniques. A good correspondence has been found between energy positions of the theoretical density of states features and structure in the constant initial state (CIS) and energy distribution curves (EDC's) for the conduction and the valence bands respectively. In the energy region between — 8 eV and 15 eV from the top of the valence band we observe four valence band and six conduction band peaks.     

A local framework for calculating coupled cluster singles and doubles excitation energies (LoFEx-CCSD)

ABSTRACT

The recently developed Local Framework for calculating Excitation energies (LoFEx) is extended to the coupled cluster singles and doubles (CCSD) model. In the new scheme, a standard CCSD excitation energy calculation is carried out within a reduced excitation orbital space (XOS), which is composed of localised molecular orbitals and natural transition orbitals determined from time-dependent Hartree–Fock theory. The presented algorithm uses a series of reduced second-order approximate coupled cluster singles and doubles (CC2) calculations to optimise the XOS in a black-box manner. This ensures that the requested CCSD excitation energies have been determined to a predefined accuracy compared to a conventional CCSD calculation. We present numerical LoFEx-CCSD results for a set of medium-sized organic molecules, which illustrate the black-box nature of the approach and the computational savings obtained for transitions that are local compared to the size of the molecule. In fact, for such local transitions, the LoFEx-CCSD scheme can be applied to molecular systems where a conventional CCSD implementation is intractable.     

Nature of heavy quasiparticles in magnetically ordered heavy fermions UPd(2)Al(3) and UPt(3)

The optical conductivity of the heavy fermions UPd(2)Al(3) and UPt(3) has been measured in the energy range from 0.04 to 5 meV. In both compounds a well pronounced pseudogap of less than 1 meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective mass and scattering rate we conclude that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the prerequisite to the formation of the heavy quasiparticles and eventually to superconductivity.     

Effect of pressure on the secondary relaxation in a simple glass former

We have studied dielectric spectra of the glass-forming liquid metafluoroaniline under hydrostatic pressure up to 700 MPa. Its glass transition pressure p(g) increases approximately linearly with temperature. Above p(g)(T), a well pronounced secondary relaxation, the Johari beta peak, is observed showing activated behavior. The activation energy rises proportionally to pressure and, consequently, proportionally to the glass transition temperature T(g)(p). The activation volume is independent of temperature but exhibits different values for pressures higher and lower than the pressure where the liquid left the ergodic regime. The activation volumes are about 1/10 and 1/6 of the molecular volume of fluoroaniline, respectively, suggesting that there are two different species of clusters.     

Measurement of the D(s)+ lifetime

A high statistics measurement of the D(s)+ lifetime from the Fermilab fixed-target FOCUS photoproduction experiment is presented. We describe the analysis of the two decay modes, D(s)+ --> phi(1020)pi+ and D(s)+ -->K*(892)0K+, used for the measurement. The measured lifetime is 507.4 +/- 5.5(stat) +/- 5.1(syst) fs using 8961 +/- 105 D(s)+ --> phi(1020)pi+ and 4680 +/- 90 D(s)+ --> K*(892)0K+ decays. This is a significant improvement over the present world average.     

Conversion of homocysteine to methionine by methionine synthase: a density functional study

This communication reports a theoretical study of the conversion of homocysteine to methionine by methionine synthase. The reaction pathway is based on density functional calculations with large basis sets, including thermodynamic, relativistic, and solvent effects. We find that the suggested SN2 mechanism explains well the experimentally observed reaction rate. The results show that the reaction is highly polar, as reflected in the change of charge density along the reaction coordinate. It is enhanced in the protein by two effects: deprotonation of the bound substrate and desolvation of substrate and cofactor in the rate-determining step.     

Interference study of the chi c0(13P0) in the reaction -pp-->pi0pi0

Fermilab experiment E835 has observed (-)pp annihilation production of the charmonium state chi(c0) and its subsequent decay into pi(0)pi(0). Although the resonant amplitude is an order of magnitude smaller than that of the nonresonant continuum production of pi(0)pi(0), an enhanced interference signal is evident. A partial wave expansion is used to extract physics parameters. The amplitudes J=0 and 2, of comparable strength, dominate the expansion. Both are accessed by L=1 in the entrance (-)pp channel. The product of the input and output branching fractions is determined to be B((-)pp-->chi(c0))xB(chi(c0)-->pi(0)pi(0))=(5.09+/-0.81+/-0.25)x10(-7).     

Growth kinetics of Si-molecular beam epitaxy

Growth mode, surface morphology, crystal perfection and growth rate of siliconmolecular beam epitaxy films were observed as function of temperature (450°–950°C), silicon flux density (8×10¹⁴–8×10¹⁵cm²/s) and surface orientation (111, 110, 100). Within the varied parameters growth proceeds by the two-dimensional growth mode via the lateral motion of atomic steps originating from the slight misorientation of commercially available substrates (typically 0.25°). Single crystalline films with high lattice perfection and smooth surfaces result from this growth mode. The growth rate — linearly dependent on Si-flux density and independant of temperature and orientation — indicates a condensation coefficient near unity. An atomic step flow model on the basis of the Burton-Cabrera-Frank theory explains this behaviour by mobile adatoms with low activation energy of diffusion.     

Field Equations of TREDER's Gravitational Theory which are Derivable from a Variational Principle. III

In TREDER 's theory of gravitation the active gravitational (SCHWARZSCHILD ) mass is, generally speaking, different from the inertial (rest) mass of a stationary space-bounded field producing system. We consider a certain class of field equations and show that the relative deviation of the inertial (rest) mass from the SCHWARZSCHILD ian mass is very small (≤1/15).