486 research outputs found
Long-range-corrected hybrids including RPA correlation
We recently demonstrated a connection between the random phase approximation
(RPA) and coupled cluster theory [J. Chem. Phys. 129, 231101 (2008)]. Based on
this result, we here propose and test a simple scheme for introducing
long-range RPA correlation into density functional theory. Our method provides
good thermochemical results and models van derWaals interactions accurately.Comment: Accepted version of the manuscrip
Actinide chemistry using singlet-paired coupled cluster and its combinations with density functionals
Singlet-paired coupled cluster doubles (CCD0) is a simplification of CCD that
relinquishes a fraction of dynamic correlation in order to be able to describe
static correlation. Combinations of CCD0 with density functionals that recover
specifically the dynamic correlation missing in the former have also been
developed recently. Here, we assess the accuracy of CCD0 and CCD0+DFT (and
variants of these using Brueckner orbitals) as compared to well-established
quantum chemical methods for describing ground-state properties of singlet
actinide molecules. The actinyl series (UO, NpO,
PuO), the isoelectronic NUN, and Thorium (ThO, ThO) and
Nobelium (NoO, NoO) oxides are studied.Comment: 8 page
Electron correlation in solids via density embedding theory
Density matrix embedding theory (Phys. Rev. Lett. 109, 186404 (2012)) and
density embedding theory ((Phys. Rev. B 89, 035140 (2014)) have recently been
introduced for model lattice Hamiltonians and molecular systems. In the present
work, the formalism is extended to the ab initio description of infinite
systems. An appropriate definition of the impurity Hamiltonian for such systems
is presented and demonstrated in cases of 1, 2 and 3 dimensions, using coupled
cluster theory as the impurity solver. Additionally, we discuss the challenges
related to disentanglement of fragment and bath states. The current approach
yields results comparable to coupled cluster calculations of infinite systems
even when using a single unit cell as the fragment. The theory is formulated in
the basis of Wannier functions but it does not require separate localization of
unoccupied bands. The embedding scheme presented here is a promising way of
employing highly accurate electronic structure methods for extended systems at
a fraction of their original computational cost
Density-based mixing parameter for hybrid functionals
A very popular ab-initio scheme to calculate electronic properties in solids
is the use of hybrid functionals in density functional theory (DFT) that mixes
a portion of Fock exchange with DFT functionals. In spite of their success, a
major problem still remains, related to the use of one single mixing parameter
for all materials. Guided by physical arguments that connect the mixing
parameter to the dielectric properties of the solid, and ultimately to its band
gap, we propose a method to calculate this parameter from the electronic
density alone. This method is able to cut significantly the error of
traditional hybrid functionals for large and small gap materials, while
retaining a good description of structural properties. Moreover, its
implementation is simple and leads to a negligible increase of the
computational time.Comment: submitte
Gaussian approximations for the exchange-energy functional of current-carrying states: Applications to two-dimensional systems
Electronic structure calculations are routinely carried out within the
framework of density-functional theory, often with great success. For electrons
in reduced dimensions, however, there is still a need for better approximations
to the exchange-correlation energy functional. Furthermore, the need for
properly describing current-carrying states represents an additional challenge
for the development of approximate functionals. In order to make progress along
these directions, we show that simple and efficient expressions for the
exchange energy can be obtained by considering the short-range behavior of the
one-body spin-density matrix. Applications to several two-dimensional systems
confirm the excellent performance of the derived approximations, and verify the
gauge-invariance requirement to be of great importance for dealing with
current-carrying states
C(240)-----The most Chemically Inert Fullerene?
The reactivity of the fullerenes is primarily a function of their strain, as measured by the pyramidalization angle or curvature of the conjugated carbon atoms. The development of faceting in the structure of large icosahedral fullerenes leads to a minimum in the value of the maximum fullerene pyramidalization angle that lies in the vicinity of C-240. On this basis it is argued that C-240 will be the most chemically inert fullerene. This observation explains the production of [10,10] single-walled nanotubes because a C-240 hemisphere is required for the nucleation of such tubes
Molecules in clusters: the case of planar LiBeBCNOF built from a triangular form LiOB and a linear four-center species FBeCN
Krueger some years ago proposed a cluster LiBeBCNOF, now called periodane.
His ground-state isomer proposal has recently been refined by Bera et al. using
DFT. Here, we take the approach of molecules in such a cluster as starting
point. We first study therefore the triangular molecule LiOB by coupled cluster
theory (CCSD) and thereby specify accurately its equilibrium geometry in free
space. The second fragment we consider is FBeCN, but treated now by restricted
Hartree-Fock (RHF) theory. This four-center species is found to be linear, and
the bond lengths are obtained from both RHF and CCSD calculations. Finally, we
bring these two entities together and find that while LiOB remains largely
intact, FBeCN becomes bent by the interaction with LiOB. Hartree-Fock and CCSD
theories then predict precisely the same lowest isomer found by Bera et al.
solely on the basis of DFT.Comment: to appear in Phys. Lett.
Multi-Determinant Wave-functions in Quantum Monte Carlo
Quantum Monte Carlo (QMC) methods have received considerable attention over
the last decades due to their great promise for providing a direct solution to
the many-body Schrodinger equation in electronic systems. Thanks to their low
scaling with number of particles, QMC methods present a compelling competitive
alternative for the accurate study of large molecular systems and solid state
calculations. In spite of such promise, the method has not permeated the
quantum chemistry community broadly, mainly because of the fixed-node error,
which can be large and whose control is difficult. In this Perspective, we
present a systematic application of large scale multi-determinant expansions in
QMC, and report on its impressive performance with first row dimers and the 55
molecules of the G1 test set. We demonstrate the potential of this strategy for
systematically reducing the fixed-node error in the wave function and for
achieving chemical accuracy in energy predictions. When compared to traditional
quantum chemistry methods like MP2, CCSD(T), and various DFT approximations,
the QMC results show a marked improvement over all of them. In fact, only the
explicitly-correlated CCSD(T) method with a large basis set produces more
accurate results. Further developments in trial wave functions and algorithmic
improvements appear promising for rendering QMC as the benchmark standard in
large electronic systems
Entanglement of electrons in interacting molecules
Quantum entanglement is a concept commonly used with reference to the
existence of certain correlations in quantum systems that have no classical
interpretation. It is a useful resource to enhance the mutual information of
memory channels or to accelerate some quantum processes as, for example, the
factorization in Shor's Algorithm. Moreover, entanglement is a physical
observable directly measured by the von Neumann entropy of the system. We have
used this concept in order to give a physical meaning to the electron
correlation energy in systems of interacting electrons. The electronic
correlation is not directly observable, since it is defined as the difference
between the exact ground state energy of the many--electrons Schroedinger
equation and the Hartree--Fock energy. We have calculated the correlation
energy and compared with the entanglement, as functions of the nucleus--nucleus
separation using, for the hydrogen molecule, the Configuration Interaction
method. Then, in the same spirit, we have analyzed a dimer of ethylene, which
represents the simplest organic conjugate system, changing the relative
orientation and distance of the molecules, in order to obtain the configuration
corresponding to maximum entanglement.Comment: 15 pages, 7 figures, standard late
- …