35,644 research outputs found
Optimized Jastrow-Slater wave functions for ground and excited states: Application to the lowest states of ethene
A quantum Monte Carlo method is presented for determining multi-determinantal
Jastrow-Slater wave functions for which the energy is stationary with respect
to the simultaneous optimization of orbitals and configuration interaction
coefficients. The approach is within the framework of the so-called energy
fluctuation potential method which minimizes the energy in an iterative fashion
based on Monte Carlo sampling and a fitting of the local energy fluctuations.
The optimization of the orbitals is combined with the optimization of the
configuration interaction coefficients through the use of additional single
excitations to a set of external orbitals. A new set of orbitals is then
obtained from the natural orbitals of this enlarged configuration interaction
expansion. For excited states, the approach is extended to treat the average of
several states within the same irreducible representation of the pointgroup of
the molecule. The relationship of our optimization method with the stochastic
reconfiguration technique by Sorella et al. is examined. Finally, the
performance of our approach is illustrated with the lowest states of ethene, in
particular with the difficult case of the singlet 1B_1u state.Comment: 12 pages, 2 figure
Fast construction of the Kohn--Sham response function for molecules
The use of the LCAO (Linear Combination of Atomic Orbitals) method for
excited states involves products of orbitals that are known to be linearly
dependent. We identify a basis in the space of orbital products that is local
for orbitals of finite support and with a residual error that vanishes
exponentially with its dimension. As an application of our previously reported
technique we compute the Kohn--Sham density response function for a
molecule consisting of atoms in operations, with
the number of frequency points. We test our construction of
by computing molecular spectra directly from the equations of
Petersilka--Gossmann--Gross in operations rather than from
Casida's equations which takes operations. We consider the good
agreement with previously calculated molecular spectra as a validation of our
construction of . Ongoing work indicates that our method is well
suited for the computation of the GW self-energy and we
expect it to be useful in the analysis of exitonic effects in molecules
Characterization of Excited States in Time-Dependent Density Functional Theory Using Localized Molecular Orbitals
Localized molecular orbitals are often used for the analysis of chemical
bonds, but they can also serve to efficiently and comprehensibly compute linear
response properties. While conventional canonical molecular orbitals provide an
adequate basis for the treatment of excited states, a chemically meaningful
identification of the different excited-state processes is difficult within
such a delocalized orbital basis. In this work, starting from an initial set of
supermolecular canonical molecular orbitals, we provide a simple one-step
top-down embedding procedure for generating a set of orbitals which are
localized in terms of the supermolecule, but delocalized over each subsystem
composing the supermolecule. Using an orbital partitioning scheme based on such
sets of localized orbitals, we further present a procedure for the construction
of local excitations and charge-transfer states within the linear response
framework of time-dependent density functional theory (TDDFT). This procedure
provides direct access to approximate diabatic excitation energies and, under
the Tamm--Dancoff approximation, also their corresponding electronic couplings
-- quantities that are of primary importance in modelling energy transfer
processes in complex biological systems. Our approach is compared with a
recently developed diabatization procedure based on subsystem TDDFT using
projection operators, which leads to a similar set of working equations.
Although both of these methods differ in the general localization strategies
adopted and the type of basis functions (Slaters vs. Gaussians) employed, an
overall decent agreement is obtained
Electrically driven photon emission from individual atomic defects in monolayer WS2.
Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources
Excited State Specific Multi-Slater Jastrow Wave Functions
We combine recent advances in excited state variational principles, fast
multi-Slater Jastrow methods, and selective configuration interaction to create
multi-Slater Jastrow wave function approximations that are optimized for
individual excited states. In addition to the Jastrow variables and linear
expansion coefficients, this optimization includes state-specific orbital
relaxations in order to avoid the compromises necessary in state-averaged
approaches. We demonstrate that, when combined with variance matching to help
balance the quality of the approximation across different states, this approach
delivers accurate excitation energies even when using very modest multi-Slater
expansions. Intriguingly, this accuracy is maintained even when studying a
difficult chlorine-anion-to- charge transfer in which traditional
state-averaged multi-reference methods must contend with different states that
require drastically different orbital relaxations.Comment: 16 pages, 6 figures, 2 table
Attosecond spectroscopy reveals alignment dependent core-hole dynamics in the ICl molecule.
The removal of electrons located in the core shells of molecules creates transient states that live between a few femtoseconds to attoseconds. Owing to these short lifetimes, time-resolved studies of these states are challenging and complex molecular dynamics driven solely by electronic correlation are difficult to observe. Here, we obtain few-femtosecond core-excited state lifetimes of iodine monochloride by using attosecond transient absorption on iodine 4d-16p transitions around 55 eV. Core-level ligand field splitting allows direct access of excited states aligned along and perpendicular to the ICl molecular axis. Lifetimes of 3.5 ± 0.4 fs and 4.3 ± 0.4 fs are obtained for core-hole states parallel to the bond and 6.5 ± 0.6 fs and 6.9 ± 0.6 fs for perpendicular states, while nuclear motion is essentially frozen on this timescale. Theory shows that the dramatic decrease of lifetime for core-vacancies parallel to the covalent bond is a manifestation of non-local interactions with the neighboring Cl atom of ICl
Density Functional Extension to Excited-State Mean-Field Theory.
We investigate an extension of excited-state mean-field theory in which the energy expression is augmented with density functional components in an effort to include the effects of weak electron correlations. The approach remains variational and entirely time independent, allowing it to avoid some of the difficulties associated with linear response and the adiabatic approximation. In particular, all of the electrons' orbitals are relaxed state specifically, and there is no reliance on Kohn-Sham orbital energy differences, both of which are important features in the context of charge transfer. Preliminary testing shows clear advantages for single-component charge transfer states, but the method, at least in its current form, is less reliable for states in which multiple particle-hole transitions contribute significantly
Is it possible to construct excited-state energy functionals by splitting k-space?
We show that our procedure of constructing excited-state energy functionals
by splitting k-space, employed so far to obtain exchange energies of
excited-states, is quite general. We do so by applying the same method to
construct modified Thomas-Fermi kinetic energy functional and its gradient
expansion up to the second order for the excited-states. We show that the
resulting kinetic energy functional has the same accuracy for the
excited-states as the ground-state functionals do for the ground-states.Comment: 20 pages, 1 figur
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