50 research outputs found
On the calculation of complete dissociation curves of closed-shell pseudo-onedimensional systems through the multireference method of increments
The Method of Increments (MoI) has been employed using a multireference
approach to calculate the dissociation curve of beryllium ring-shaped clusters
Be of different sizes. Benchmarks obtained through different single and
multireference methods including the ab initio Density Matrix Renormalization
Group (DMRG) were used to verify the validity of the MoI truncation which
showed a reliable behavior for the whole dissociation curve. Moreover we
investigated the size dependence of the correlation energy at different
distances in order to extrapolate the values for the periodic chain and to
discuss the transition from a metal-like to a insulating-like behavior of the
wave function through quantum chemical considerations
H\"uckel--Hubbard-Ohno modeling of -bonds in ethene and ethyne with application to trans-polyacetylene
Quantum chemistry calculations provide the potential energy between two
carbon atoms in ethane (HCCH), ethene (HCCH), and ethyne
(HCCH) as a function of the atomic distance. Based on the energy
function for the -bond in ethane, , we use the H\"uckel
model with Hubbard--Ohno interaction for the ~electrons to describe the
energies and for the
double bond in ethene and the triple bond in ethyne,
respectively. The fit of the force functions shows that the Peierls coupling
can be estimated with some precision whereas the Hubbard-Ohno parameters are
insignificant at the distances under consideration. We apply the
H\"uckel-Hubbard-Ohno model to describe the bond lengths and the energies of
elementary electronic excitations of trans-polyacetylene, (CH), and adjust
the -bond potential for conjugated polymers.Comment: 10 pages, 7 figures, 3 table
Predicting the FCI energy of large systems to chemical accuracy from restricted active space density matrix renormalization group calculations
We theoretically derive and validate with large scale simulations a
remarkably accurate power law scaling of errors for the restricted active space
density matrix renormalization group (DMRG-RAS) method [arXiv:2111.06665] in
electronic structure calculations. This yields a new extrapolation method,
DMRG-RAS-X, which reaches chemical accuracy for strongly correlated systems
such as the Chromium dimer, dicarbon up to a large cc-pVQZ basis, and even a
large chemical complex like the FeMoco with significantly lower computational
demands than previous methods. The method is free of empirical parameters,
performed robustly and reliably in all examples we tested, and has the
potential to become a vital alternative method for electronic structure
calculations in quantum chemistry, and more generally for the computation of
strong correlations in nuclear and condensed matter physics.Comment: 16 pages, 10 figure
Dissecting the Bond Formation Process of -Metal-Ethene Complexes with Multireference Approaches
The bonding mechanism of ethene to a nickel or palladium center is studied by
the density matrix renormalization group algorithm, the complete active space
self consistent field method, coupled cluster theory, and density functional
theory. Specifically, we focus on the interaction between the metal atom and
bis-ethene ligands in perpendicular and parallel orientations. The bonding
situation in these structural isomers is further scrutinized using energy
decomposition analysis and quantum information theory. Our study highlights the
fact that when two ethene ligands are oriented perpendicular to each other, the
complex is stabilized by the metal-to-ligand double-back-bonding mechanism.
Moreover, we demonstrate that nickel-ethene complexes feature a stronger and
more covalent interaction between the ligands and the metal center than
palladium-ethene compounds with similar coordination spheres.Comment: 13 pages, 9 figure
A simple electronic ladder model harboring parafermions
Parafermions are anyons with the potential for realizing non-local qubits
that are resilient to local perturbations. Compared to Majorana zero modes,
braiding of parafermions implements an extended set of topologically protected
quantum gates. This, however, comes at the price that parafermionic zero modes
can not be realized in the absence of strong interactions whose theoretical
description is challenging. In the present work, we construct a simple lattice
model for interacting spinful electrons with parafermionic zero energy modes.
The explicit microscopic nature of the considered model highlights new
realization avenues for these exotic excitations in recently fabricated quantum
dot arrays. By density matrix renormalization group calculations, we identify a
broad range of parameters, with well-localized zero modes, whose parafermionic
nature is substantiated by their unique periodic Josephson spectrum
The correlation theory of the chemical bond
The quantum mechanical description of the chemical bond is generally given in terms of delocalized bonding orbitals, or, alternatively, in terms of correlations of occupations of localised orbitals. However, in the latter case, multiorbital correlations were treated only in terms of two-orbital correlations, although the structure of multiorbital correlations is far richer; and, in the case of bonds established by more than two electrons, multiorbital correlations represent a more natural point of view. Here, for the first time, we introduce the true multiorbital correlation theory, consisting of a framework for handling the structure of multiorbital correlations, a toolbox of true multiorbital correlation measures, and the formulation of the multiorbital correlation clustering, together with an algorithm for obtaining that. These make it possible to characterise quantitatively, how well a bonding picture describes the chemical system. As proof of concept, we apply the theory for the investigation of the bond structures of several molecules. We show that the non-existence of well-defined multiorbital correlation clustering provides a reason for debated bonding picture
Orbital entanglement in bond-formation processes
The accurate calculation of the (differential) correlation energy is central
to the quantum chemical description of bond-formation and bond-dissociation
processes. In order to estimate the quality of single- and multi-reference
approaches for this purpose, various diagnostic tools have been developed. In
this work, we elaborate on our previous observation [J. Phys. Chem. Lett. 3,
3129 (2012)] that one- and two-orbital-based entanglement measures provide
quantitative means for the assessment and classification of electron
correlation effects among molecular orbitals. The dissociation behavior of some
prototypical diatomic molecules features all types of correlation effects
relevant for chemical bonding. We demonstrate that our entanglement analysis is
convenient to dissect these electron correlation effects and to provide a
conceptual understanding of bond-forming and bond-breaking processes from the
point of view of quantum information theory.Comment: 42 pages, 10 figures, 11 tables, incl. supp. in