33 research outputs found
Path Integral and Solutions of the Constraint Equations: The Case of Reducible Gauge Theories
It is shown that the BRST path integral for reducible gauge theories, with
appropriate boundary conditions on the ghosts, is a solution of the constraint
equations. This is done by relating the BRST path integral to the kernel of the
evolution operator projected on the physical subspace.Comment: 10 pages Tex file, ULB-TH-94/0
On the Quantization of Reducible Gauge Systems
Reducible gauge theories with constraints linear in the momenta are
quantized. The equivalence of the reduced phase space quantization, Dirac
quantization and BRST quantization is established. The ghosts of ghosts are
found to play a crucial role in the equivalence proof.Comment: 41 pages, Plain Tex file, ULB-PMIF\92-07, GTCRG/92-0
First-principles calculations of the self-trapped exciton in crystalline NaCl
The atomic and electronic structure of the lowest triplet state of the
off-center (C2v symmetry) self-trapped exciton (STE) in crystalline NaCl is
calculated using the local-spin-density (LSDA) approximation. In addition, the
Franck-Condon broadening of the luminescence peak and the a1g -> b3u absorption
peak are calculated and compared to experiment. LSDA accurately predicts
transition energies if the initial and final states are both localized or
delocalized, but 1 eV discrepancies with experiment occur if one state is
localized and the other is delocalized.Comment: 4 pages with 4 embeddded figure
Modelling charge self-trapping in wide-gap dielectrics: Localization problem in local density functionals
We discuss the adiabatic self-trapping of small polarons within the density
functional theory (DFT). In particular, we carried out plane-wave
pseudo-potential calculations of the triplet exciton in NaCl and found no
energy minimum corresponding to the self-trapped exciton (STE) contrary to the
experimental evidence and previous calculations. To explore the origin of this
problem we modelled the self-trapped hole in NaCl using hybrid density
functionals and an embedded cluster method. Calculations show that the
stability of the self-trapped state of the hole drastically depends on the
amount of the exact exchange in the density functional: at less than 30% of the
Hartree-Fock exchange, only delocalized hole is stable, at 50% - both
delocalized and self-trapped states are stable, while further increase of exact
exchange results in only the self-trapped state being stable. We argue that the
main contributions to the self-trapping energy such as the kinetic energy of
the localizing charge, the chemical bond formation of the di-halogen quasi
molecule, and the lattice polarization, are represented incorrectly within the
Kohn-Sham (KS) based approaches.Comment: 6 figures, 1 tabl
Ab initio study of the F centers in CaF_2: calculations of the optical absorption, diffusion and binding energies
Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal