96 research outputs found
First-Principles Studies of Local Order in Relaxor Ferroelectrics
A key to optimizing the growth of the new single-crystal relaxor
ferroelectrics is resolving basic questions concerning their structural
properties and energetics. We report on initial first-principles total energy
and force calculations, examining the energetics of local order in PZN type
relaxors.Comment: 9 pages, LaTeX (aipproc), three eps figure
Auxiliary-field quantum Monte Carlo study of first- and second-row post-d elements
A series of calculations for the first- and second-row post-d elements (Ga-Br
and In-I) are presented using the phaseless auxiliary-field quantum Monte Carlo
(AF QMC) method. This method is formulated in a Hilbert space defined by any
chosen one-particle basis, and maps the many-body problem into a linear
combination of independent-particle solutions with external auxiliary fields.
The phase/sign problem is handled approximately by the phaseless formalism
using a trial wave function, which in our calculations was chosen to be the
Hartree-Fock solution. We used the consistent correlated basis sets of Peterson
and coworkers, which employ a small core relativistic pseudopotential. The AF
QMC results are compared with experiment and with those from density-functional
(GGA and B3LYP) and coupled-cluster CCSD(T) calculations. The AF QMC total
energies agree with CCSD(T) to within a few milli-hartrees across the systems
and over several basis sets. The calculated atomic electron affinities,
ionization energies, and spectroscopic properties of dimers are, at large basis
sets, in excellent agreement with experiment.Comment: 10 pages, 2 figures. To be published in Journal of Chemical Physic
Electron-phonon coupling and exchange-correlation effects in superconducting H3S under high pressure
We investigate the H3S phase of sulfur hydride under high pressure similar or equal to 200 GPa by means of ab initio calculations within the framework of the density-functional theory with the PBE0 hybrid exchange-correlation (E-xc) approximation. The choice of E-xc has the largest effect on the calculated electron-phonon coupling (EPC) matrix elements; the high-pressure equation of state and phonon frequencies are only slightly modified. Mode-dependent EPC correction factors are determined from PBE0 using a frozen-phonon supercell approach, while standard density-functional perturbation theory is used to determine the EPC with PBE generalized-gradient approximation E-xc. Our principle finding is that the calculated PBE0 T-c is enhanced by 25% compared to PBE. This is similar in magnitude, but in opposite direction, to the proposed suppression of T-c by anharmonic effects [I. Errea et al., Phys. Rev. Lett. 114, 157004 (2015)]. Our calculations demonstrate the importance of considering exchange-correlation approximations for calculations of superconducting properties for this class of materials
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