51 research outputs found
Combining Density Functional Theory and Density Matrix Functional Theory
We combine density-functional theory with density-matrix functional theory to
get the best of both worlds. This is achieved by range separation of the
electronic interaction which permits to rigorously combine a short-range
density functional with a long-range density-matrix functional. The short-range
density functional is approximated by the short-range version of the
Perdew-Burke-Ernzerhof functional (srPBE). The long-range density-matrix
functional is approximated by the long-range version of the Buijse-Baerends
functional (lrBB). The obtained srPBE+lrBB method accurately describes both
static and dynamic electron correlation at a computational cost similar to that
of standard density-functional approximations. This is shown for the
dissociation curves of the H, LiH, BH and HF molecules.Comment: 4 pages, 5 figure
Ensemble density variational methods with selfand ghost-interaction-corrected functionals
Ensemble density functional theory (DFT) offers a way of predicting excited-states energies of atomic and molecular systems without referring to a density response function. Despite a significant theoretical work, practical applications of the proposed approximations have been scarce and they do not allow for a fair judgement of the potential usefulness of ensemble DFT with available functionals. In the paper, we investigate two forms of ensemble density functionals formulated within ensemble DFT framework: the Gross, Oliveira, and Kohn (GOK) functional proposed by Gross et al. [Phys. Rev. A37, 2809 (1988)] alongside the orbital-dependent eDFT form of the functional introduced by Nagy [J. Phys. B34, 2363 (2001)] (the acronym eDFT proposed in analogy to eHF – ensemble Hartree-Fock method). Local and semi-local ground-state density functionals are employed in both approaches. Approximate ensemble density functionals contain not only spurious self-interaction but also the so-called ghost-interaction which has no counterpart in the ground-state DFT. We propose how to correct the GOK functional for both kinds of interactions in approximations that go beyond the exact-exchange functional. Numerical applications lead to a conclusion that functionals free of the ghost-interaction by construction, i.e., eDFT, yield much more reliable results than approximate self- and ghost-interaction-corrected GOK functional. Additionally, local density functional corrected for self-interaction employed in the eDFT framework yields excitations energies of the accuracy comparable to that of the uncorrected semi-local eDFT functional
W centrum doskonałości
Artykuł zamieszczony jest w : Życie Uczelni : biuletyn informacyjny Politechniki Łódzkiej, nr 154, grudzień 2020Politechnika Łódzka jest jednym z partnerów Centrum Doskonałości TREX, finansowanego
w ramach programu Horyzont 2020. Projekt potrwa 36 miesięcy.
Jego całkowity budżet wynosi 5 mln euro
Toward more accurate adiabatic connection approach for multireference wave functions
A multiconfigurational adiabatic connection (AC) formalism is an attractive
approach to computing dynamic correlation within CASSCF and DMRG models.
Practical realizations of AC have been based on two approximations: i) fixing
one- and two-electron reduced density matrices (1- and 2-RDMs) at the
zero-coupling constant limit and ii) extended random phase approximation
(ERPA). This work investigates the the effect of removing the "fixed-RDM"
approximation in AC. The analysis is carried out for two electronic Hamiltonian
partitionings: the group product function- and the Dyall-Hamiltonians. Exact
reference AC integrands are generated from the DMRG FCI solver. Two AC models
are investigated, employing either exact 1- and 2-RDMs or their second-order
expansions in the coupling constant in the ERPA equations. Calculations for
model molecules indicate that lifting the fixed-RDM approximation is a viable
way toward improving accuracy of the existing AC approximations
Variational quantum eigensolver boosted by adiabatic connection
In this work we integrate the variational quantum eigensolver (VQE) with the
adiabatic connection (AC) method for efficient simulations of chemical problems
on near-term quantum computers. Orbital optimized VQE methods are employed to
capture the strong correlation within an active space and classical AC
corrections recover the dynamical correlation effects comprising electrons
outside of the active space. On two challenging strongly correlated problems,
namely the dissociation of N and the electronic structure of the
tetramethyleneethane biradical, we show that the combined VQE-AC approach
enhances the performance of VQE dramatically. Moreover, since the AC
corrections do not bring any additional requirements on quantum resources or
measurements, they can literally boost the VQE algorithms. Our work paves the
way towards quantum simulations of real-life problems on near-term quantum
computers
How accurate is the strongly orthogonal geminal theory in predicting excitation energies? Comparison of the extended random phase approximation and the linear response theory approaches
Performance of the antisymmetrized product of strongly orthogonal geminal (APSG) ansatz in describing ground states of molecules has been extensively explored in the recent years. Not much is known, however, about possibilities of obtaining excitation energies from methods that would rely on the APSG ansatz. In the paper we investigate the recently proposed extended random phase approximations, ERPA and ERPA2, that employ APSG reduced density matrices. We also propose a time-dependent linear response APSG method (TD-APSG). Its relation to the recently proposed phase including natural orbital theory is elucidated. The methods are applied to Li2, BH, H2O, and CH2O molecules at equilibrium geometries and in the dissociating limits. It is shown that ERPA2 and TD-APSG perform better in describing double excitations than ERPA due to inclusion of the so-called diagonal double elements. Analysis of the potential energy curves of Li2, BH, and H2O reveals that ERPA2 and TD-APSG describe correctly excitation energies of dissociating molecules if orbitals involved in breaking bonds are involved. For single excitations of molecules at equilibrium geometries the accuracy of the APSG-based methods approaches that of the time-dependent Hartree-Fock method with the increase of the system size. A possibility of improving the accuracy of the TD-APSG method for single excitations by splitting the electron-electron interaction operator into the long- and short-range terms and employing density functionals to treat the latter is presented
The role of spin polarization and dynamic correlation in singlet-triplet gap inversion of heptazine derivatives
The new generation of proposed light-emitting molecules for OLEDs has raised
a considerable research interest due to its exceptional feature-a negative
singlet-triplet (ST) gap violating the Hund's multiplicity rule in the excited
S1 and T1 states. We investigate the role of spin polarization in the mechanism
of ST gap inversion. Spin polarization is associated with doubly excited
determinants of certain types, whose presence in the wavefunction expansion
favors the energy of the singlet state more than that of the triplet. Using a
perturbation theory-based model for spin polarization, we propose a simple
descriptor for prescreening of candidate molecules with negative ST gaps and
prove its usefulness for heptazine-type molecules. Numerical results show that
the quantitative effect of spin polarization is approximately
inverse-proportional to the HOMO-LUMO exchange integral. Comparison of single-
and multireference coupled- cluster predictions of ST gaps shows that the
former methods provide good accuracy by correctly balancing the effects of
doubly excited determinants and dynamic correlation. We also show that accurate
ST gaps may be obtained using a complete active space model supplemented with
dynamic correlation from multireference adiabatic connection theory
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