642 research outputs found
Use of isotopes for studying reaction mechanisms: 3. Secondary kinetic isotope effect
The effects of isotopic substitution on equilibrium constants and reaction rates in processes which do not directly involve the isotopic atom are described. In particular, the mechanistic details which can be obtained by quantifying the secondary kinetic isotope effect and steric isotope effect are illustrated
Laser-induced electron localization in H: Mixed quantum-classical dynamics based on the exact time-dependent potential energy surface
We study the exact nuclear time-dependent potential energy surface (TDPES)
for laser-induced electron localization with a view to eventually developing a
mixed quantum-classical dynamics method for strong-field processes. The TDPES
is defined within the framework of the exact factorization [A. Abedi, N. T.
Maitra, and E. K. U. Gross, Phys. Rev. Lett. 105, 123002 (2010)] and contains
the exact effect of the couplings to the electronic subsystem and to any
external fields within a scalar potential. We compare its features with those
of the quasistatic potential energy surfaces (QSPES) often used to analyse
strong-field processes. We show that the gauge-independent component of the
TDPES has a mean-field-like character very close to the density-weighted
average of the QSPESs. Oscillations in this component are smoothened out by the
gauge-dependent component, and both components are needed to yield the correct
force on the nuclei. Once the localization begins to set in, the gradient of
the exact TDPES tracks one QSPES and then switches to the other, similar to the
description provided by surface-hopping between QSPESs. We show that evolving
an ensemble of classical nuclear trajectories on the exact TDPES accurately
reproduces the exact dynamics. This study suggests that the mixed
quantum-classical dynamics scheme based on evolving multiple classical nuclear
trajectories on the exact TDPES will be a novel and useful method to simulate
strong field processes.Comment: 10 pages, 6 figure
Time-dependent density functional theory: Past, present, and future
Time-dependent density functional theory (TDDFT) is presently enjoying
enormous popularity in quantum chemistry, as a useful tool for extracting
electronic excited state energies. This article discusses how TDDFT is much
broader in scope, and yields predictions for many more properties. We discuss
some of the challenges involved in making accurate predictions for these
properties.Comment: 12 pages, 4 figure
A complete description of the magnetic ground state in spinel vanadates
Capturing the non-collinear magnetic ground state of the spinel vanadates
AVO (A= Mn, Fe and Co) remains an outstanding challenge for
state-of-the-art ab-initio methods. We demonstrate that both the non-collinear
spin texture, as well as the magnitude of local moments, are captured by a
single value of the on-site Hubbard of 2.7~eV in conjunction with the local
spin density approximation (LSDA+), provided the source term (i.e., magnetic
monopole term) is removed from the exchange-correlation magnetic field . We further demonstrate that the magnetic monopole structure in is highly sensitive to the value of , to the extent that the
interplay between on-site localization and local moment magnitude is
qualitatively different depending on whether the source term is removed or not.
This suggests that in treating strongly correlated magnetic materials within
the LSDA+ formalism, subtraction of the unphysical magnetic monopole term
from the exchange-correlation magnetic field is essential to correctly treat
the magnetic ground state.Comment: 4 pages, 3 figure
Location, function, and nucleotide sequence of a promoter for bacteriophage T3 RNA polymerase
The major promoters for bacteriophage T3 RNA polymerase on the T3 genome have been mapped by DNA.RNA filter hybridization. One promoter is located in a 300-base-pair Hpa I restriction fragment near the genetic "left" end of T3 DNA. The sequence in the vicinity of the major initiation site of transcription in this region has been determined. A part of the (-)strand sequence is 5' T-A-T-T-T-A-C-C-C-T-C-A-C-T-A-A-A-G-+1 G-G-A-A-U 3'. Comparison of this sequence with the prototype 23-base-pair promoter sequence for bacteriophage T7 RNA polymerase shows a striking pattern of homology and divergence. Between positions -9 and +4, the sequences are virtually identical, whereas between positions -17 and -10, the sequences are quite different. It is postulated that these sequence subsets may perform different functions in transcription initiation by the phage RNA polymerases
Excitations in time-dependent density-functional theory
An approximate solution to the time-dependent density functional theory
(TDDFT) response equations for finite systems is developed, yielding
corrections to the single-pole approximation. These explain why allowed
Kohn-Sham transition frequencies and oscillator strengths are usually good
approximations to the true values, and why sometimes they are not. The
approximation yields simple expressions for G\"orling-Levy perturbation theory
results, and a method for estimating expectation values of the unknown
exchange-correlation kernel.Comment: 4 pages, 1 tabl
Correlated electron-nuclear dynamics: Exact factorization of the molecular wavefunction
It was recently shown [A. Abedi, N. T. Maitra, and E. K. U. Gross, Phys. Rev. Lett. 105, 123002 (2010)] that the complete wavefunction for a system of electrons and nuclei evolving in a timedependent external potential can be exactly factorized into an electronic wavefunction and a nuclear wavefunction. The concepts of an exact time-dependent potential energy surface (TDPES) and exact time-dependent vector potential emerge naturally from the formalism. Here, we present a detailed description of the formalism, including a full derivation of the equations that the electronic and nuclear wavefunctions satisfy. We demonstrate the relationship of this exact factorization to the traditional Born-Oppenheimer expansion. A one-dimensional model of the H + 2 molecule in a laser field shows the usefulness of the exact TDPES in interpreting coupled electron-nuclear dynamics: we show how features of its structure indicate the mechanism of dissociation. We compare the exact TDPES with potential energy surfaces from the time-dependent Hartree-approach, and also compare traditional Ehrenfest dynamics with Ehrenfest dynamics on the exact TDPES
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