37 research outputs found
Linear interpolation method in ensemble Kohn-Sham and range-separated density-functional approximations for excited states
Gross-Oliveira-Kohn density functional theory (GOK-DFT) for ensembles is in
principle very attractive, but has been hard to use in practice. A novel,
practical model based on GOK-DFT for the calculation of electronic excitation
energies is discussed. The new model relies on two modifications of GOK-DFT:
use of range separation and use of the slope of the linearly-interpolated
ensemble energy, rather than orbital energies. The range-separated approach is
appealing as it enables the rigorous formulation of a multi-determinant
state-averaged DFT method. In the exact theory, the short-range density
functional, that complements the long-range wavefunction-based ensemble energy
contribution, should vary with the ensemble weights even when the density is
held fixed. This weight dependence ensures that the range-separated ensemble
energy varies linearly with the ensemble weights. When the (weight-independent)
ground-state short-range exchange-correlation functional is used in this
context, curvature appears thus leading to an approximate weight-dependent
excitation energy. In order to obtain unambiguous approximate excitation
energies, we propose to interpolate linearly the ensemble energy between
equiensembles. It is shown that such a linear interpolation method (LIM) can be
rationalized and that it effectively introduces weight dependence effects. As
proof of principle, LIM has been applied to He, Be, H in both equilibrium
and stretched geometries as well as the stretched HeH molecule. Very
promising results have been obtained for both single (including charge
transfer) and double excitations with spin-independent short-range local and
semi-local functionals. Even at the Kohn--Sham ensemble DFT level, that is
recovered when the range-separation parameter is set to zero, LIM performs
better than standard time-dependent DFT.Comment: 26 pages, 8 figure
Alternative separation of exchange and correlation energies in range-separated density-functional perturbation theory
An alternative separation of short-range exchange and correlation energies is
used in the framework of second-order range-separated density-functional
perturbation theory. This alternative separation was initially proposed by
Toulouse et al. [Theor. Chem. Acc. 114, 305 (2005)] and relies on a long-range
interacting wavefunction instead of the non-interacting Kohn-Sham one. When
second-order corrections to the density are neglected, the energy expression
reduces to a range-separated double-hybrid (RSDH) type of functional, RSDHf,
where "f" stands for "full-range integrals" as the regular full-range
interaction appears explicitly in the energy expression when expanded in
perturbation theory. In contrast to usual RSDH functionals, RSDHf describes the
coupling between long- and short-range correlations as an orbital-dependent
contribution. Calculations on the first four noble-gas dimers show that this
coupling has a significant effect on the potential energy curves in the
equilibrium region, improving the accuracy of binding energies and equilibrium
bond distances when second-order perturbation theory is appropriate.Comment: 5 figure
Exploration of H2 binding to the [NiFe]-hydrogenase active site with multiconfigurational density functional theory
The combination of density functional theory (DFT) with a
multiconfigurational wave function is an efficient way to include dynamical
correlation in calculations with multiconfiguration self-consistent field wave
functions. These methods can potentially be employed to elucidate reaction
mechanisms in bio-inorganic chemistry, where many other methods become either
too computationally expensive or too inaccurate. In this paper, a complete
active space (CAS) short-range DFT (CAS-srDFT) hybrid was employed to
investigate a bio-inorganic system, namely H2 binding to the active site of
[NiFe] hydrogenase. This system was previously investigated with
coupled-cluster (CC) and multiconfigurational methods in form of
cumulant-approximated second-order perturbation theory, based on the density
matrix renormalization group (DMRG). We find that it is more favorable for H2
to bind to Ni than to Fe, in agreement with previous CC and DMRG calculations.
The accuracy of CAS-srDFT is comparable to both CC and DMRG, despite that much
smaller active spaces were employed. This enhanced efficiency at smaller active
spaces shows that CAS-srDFT can become a useful method for bio-inorganic
chemistry.Comment: 22 page
Implementation of relativistic coupled cluster theory for massively parallel GPU-accelerated computing architectures
In this paper, we report a reimplementation of the core algorithms of
relativistic coupled cluster theory aimed at modern heterogeneous
high-performance computational infrastructures. The code is designed for
efficient parallel execution on many compute nodes with optional GPU
coprocessing, accomplished via the new ExaTENSOR back end. The resulting
ExaCorr module is primarily intended for calculations of molecules with one or
more heavy elements, as relativistic effects on electronic structure are
included from the outset. In the current work, we thereby focus on exact
2-component methods and demonstrate the accuracy and performance of the
software. The module can be used as a stand-alone program requiring a set of
molecular orbital coefficients as starting point, but is also interfaced to the
DIRAC program that can be used to generate these. We therefore also briefly
discuss an improvement of the parallel computing aspects of the relativistic
self-consistent field algorithm of the DIRAC program
Investigation of Multiconfigurational Short-Range Density Functional Theory for Electronic Excitations in Organic Molecules
Computational methods
that can accurately and effectively predict
all types of electronic excitations for any molecular system are missing
in the toolbox of the computational chemist. Although various Kohn–Sham
density-functional methods (KS-DFT) fulfill this aim in some cases,
they become inadequate when the molecule has near-degeneracies and/or
low-lying double-excited states. To address these issues we have recently
proposed multiconfiguration short-range density-functional theoryMC-srDFTas
a new tool in the toolbox. While initial applications for systems
with multireference character and double excitations have been promising,
it is nevertheless important that the accuracy of MC-srDFT is at least
comparable to the best KS-DFT methods also for organic molecules that
are typically of single-reference character. In this paper we therefore
systematically investigate the performance of MC-srDFT for a selected
benchmark set of electronic excitations of organic molecules, covering
the most common types of organic chromophores. This investigation
confirms the expectation that the MC-srDFT method is accurate for
a broad range of excitations and comparable to accurate wave function
methods such as CASPT2, NEVPT2, and the coupled cluster based CC2
and CC3
Excitation Spectra of Nucleobases with Multiconfigurational Density Functional Theory
Range-separated hybrid methods between
wave function theory and
density functional theory (DFT) can provide high-accuracy results,
while correcting some of the inherent flaws of both the underlying
wave function theory and DFT. We here assess the accuracy for excitation
energies of the nucleobases thymine, uracil, cytosine, and adenine,
using a hybrid between complete active space self-consistent field
(CASSCF) and DFT methods. The method is based on range separation,
thereby avoiding all double-counting of electron correlation and is
denoted long-range CASSCF short-range DFT (CAS-srDFT). Using a linear
response extension of CAS-srDFT, we compare the first 7–8 excited
states of the nucleobases with perturbative multireference approaches
as well as coupled cluster based methods. Our results show that the
CAS-srDFT method can provide accurate excitation energies in good
correspondence with the computationally more expensive methods