1,082 research outputs found
Theoretical optical and x-ray spectra of liquid and solid H_2O
Theoretical optical and x-ray spectra of model structures of water and ice
are calculated using a many-body perturbation theory, Bethe-Salpeter equation
(BSE) approach implemented in the valence- and core-excitation codes AI2NBSE
and OCEAN. These codes use ab initio density functional theory wave functions
from a plane-wave, pseudopotential code, quasi-particle self energy
corrections, and a BSE treatment of particle-hole interactions. The approach
improves upon independent-particle methods through the inclusion of a complex,
energy-dependent self energy and screened particle-hole interactions to account
for inelastic losses and excitonic effects. These many-body effects are found
to be crucial for quantitative calculations of ice and water spectra
Cumulant expansion for phonon contributions to the electron spectral function
We describe an approach for calculations of phonon contributions to the
electron spectral function, including both quasiparticle properties and
satellites. The method is based on a cumulant expansion for the retarded
one-electron Green's function and a many-pole model for the electron
self-energy. The electron-phonon couplings are calculated from the Eliashberg
functions, and the phonon density of states is obtained from a Lanczos
representation of the phonon Green's function. Our calculations incorporate ab
initio dynamical matrices and electron-phonon couplings from the density
functional theory code ABINIT. Illustrative results are presented for several
elemental metals and for Einstein and Debye models with a range of coupling
constants. These are compared with experiment and other theoretical models.
Estimates of corrections to Migdal's theorem are obtained by comparing with
leading order contributions to the self-energy, and are found to be significant
only for large electron-phonon couplings at low temperatures
Bethe-Salpeter Equation Calculations of Core Excitation Spectra
We present a hybrid approach for GW/Bethe-Salpeter Equation (BSE)
calculations of core excitation spectra, including x-ray absorption (XAS),
electron energy loss spectra (EELS), and non-resonant inelastic x-ray
scattering (NRIXS). The method is based on {\it ab initio} wavefunctions from
the plane-wave pseudopotential code ABINIT; atomic core-level states and
projector augmented wave (PAW) transition matrix elements; the NIST core-level
BSE solver; and a many-pole GW self-energy model to account for final-state
broadening and self-energy shifts. Multiplet effects are also accounted for.
The approach is implemented using an interface dubbed OCEAN (Obtaining Core
Excitations using ABINIT and NBSE). To demonstrate the utility of the code we
present results for the K-edges in LiF as probed by XAS and NRIXS, the K-edges
of KCl as probed by XAS, the Ti L_2,3-edge in SrTiO_3 as probed by XAS, and the
Mg L_2,3-edge in MgO as probed by XAS. We compare the results to experiments
and results obtained using other theoretical approaches
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