231 research outputs found
The Kinetic Energy of Hydrocarbons as a Function of Electron Density and Convolutional Neural Networks
We demonstrate a convolutional neural network trained to reproduce the
Kohn-Sham kinetic energy of hydrocarbons from electron density. The output of
the network is used as a non-local correction to the conventional local and
semi-local kinetic functionals. We show that this approximation qualitatively
reproduces Kohn-Sham potential energy surfaces when used with conventional
exchange correlation functionals. Numerical noise inherited from the
non-linearity of the neural network is identified as the major challenge for
the model. Finally we examine the features in the density learned by the neural
network to anticipate the prospects of generalizing these models
Orbital optimization in the perfect pairing hierarchy. Applications to full-valence calculations on linear polyacenes
We describe the implementation of orbital optimization for the models in the
perfect pairing hierarchy [Lehtola et al, J. Chem. Phys. 145, 134110 (2016)].
Orbital optimization, which is generally necessary to obtain reliable results,
is pursued at perfect pairing (PP) and perfect quadruples (PQ) levels of theory
for applications on linear polyacenes, which are believed to exhibit strong
correlation in the {\pi} space. While local minima and {\sigma}-{\pi} symmetry
breaking solutions were found for PP orbitals, no such problems were
encountered for PQ orbitals. The PQ orbitals are used for single-point
calculations at PP, PQ and perfect hextuples (PH) levels of theory, both only
in the {\pi} subspace, as well as in the full {\sigma}{\pi} valence space. It
is numerically demonstrated that the inclusion of single excitations is
necessary also when optimized orbitals are used. PH is found to yield good
agreement with previously published density matrix renormalization group (DMRG)
data in the {\pi} space, capturing over 95% of the correlation energy.
Full-valence calculations made possible by our novel, efficient code reveal
that strong correlations are weaker when larger bases or active spaces are
employed than in previous calculations. The largest full-valence PH
calculations presented correspond to a (192e,192o) problem.Comment: 19 pages, 4 figure
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Modeling Coherent Anti-Stokes Raman Scattering with Time-Dependent Density Functional Theory: Vacuum and Surface Enhancement
We present the first density functional simulations of coherent anti-Stokes Raman scattering (CARS) and an analysis of the chemical effects upon binding to a metal surface. Spectra are obtained from first-principles electronic structure calculations and are compared with available experiments and previously available theoretical results following from Hartree–Fock polarizability derivatives. A first approximation to the nonresonant portion of the CARS signal is also explored. We examine the silver pyridine cluster models of the surface chemical signal enhancement, previously introduced for surface-enhanced Raman scattering. Chemical resonant intensity enhancements of roughly are found for several model clusters. The prospects of realizing further enhancement of CARS signal with metal surfaces is discussed in light of the predicted chemical enhancements.Chemistry and Chemical Biolog
Exciton coherence lifetimes from electronic structure
We model the coherent energy transfer of an electronic excitation within
covalently linked aromatic homodimers from first-principles, to answer whether
the usual models of the bath calculated via detailed electronic structure
calculations can reproduce the key dynamics. For these systems the timescales
of coherent transport are experimentally known from time-dependent polarization
anisotropy measurements, and so we can directly assess the whether current
techniques might be predictive for this phenomenon. Two choices of electronic
basis states are investigated, and their relative merits discussed regarding
the predictions of the perturbative model. The coupling of the electronic
degrees of freedom to the nuclear degrees of freedom is calculated rather than
assumed, and the fluorescence anisotropy decay is directly reproduced.
Surprisingly we find that although TDDFT absolute energies are routinely in
error by orders of magnitude more than the coupling energy, the coherent
transport properties of these dimers can be semi-quantitatively reproduced from
first-principles. The directions which must be pursued to yield predictive and
reliable prediction of coherent transport are suggested.Comment: 22 pages, 7 figure
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