256 research outputs found
Electron paramagnetic resonance g-tensors from state interaction spin-orbit coupling density matrix renormalization group
We present a state interaction spin-orbit coupling method to calculate
electron paramagnetic resonance (EPR) -tensors from density matrix
renormalization group wavefunctions. We apply the technique to compute
-tensors for the \ce{TiF3} and \ce{CuCl4^2-} complexes, a [2Fe-2S] model of
the active center of ferredoxins, and a \ce{Mn4CaO5} model of the S2 state of
the oxygen evolving complex. These calculations raise the prospects of
determining -tensors in multireference calculations with a large number of
open shells.Comment: 19 page
Tensor factorizations of local second-order M{\o}ller Plesset theory
Efficient electronic structure methods can be built around efficient tensor
representations of the wavefunction. Here we describe a general view of tensor
factorization for the compact representation of electronic wavefunctions. We
use these ideas to construct low-complexity representations of the doubles
amplitudes in local second order M{\o}ller-Plesset perturbation theory. We
introduce two approximations - the direct orbital specific virtual
approximation and the full orbital specific virtual approximation. In these
approximations, each occupied orbital is associated with a small set of
correlating virtual orbitals. Conceptually, the representation lies between the
projected atomic orbital representation in Pulay-Saeb{\o} local correlation
theories and pair natural orbital correlation theories. We have tested the
orbital specific virtual approximations on a variety of systems and properties
including total energies, reaction energies, and potential energy curves.
Compared to the Pulay-Saeb{\o} ansatz, we find that these approximations
exhibit favourable accuracy and computational times, while yielding smooth
potential energy curves
Perfect Reflection of Chiral Fermions in Gated Graphene Nanoribbons
We describe the results of a theoretical study of transport through gated
metallic graphene nanoribbons using a non-equilibrium Green function method.
Although analogies with quantum field theory predict perfect transmission of
chiral fermions through gated regions in one dimension, we find \emph{perfect
reflection} of chiral fermions in armchair ribbons for specific configurations
of the gate. This effect should be measurable in narrow graphene constrictions
gated by a charged carbon nanotube.Comment: 9 pages, 3 figures. Submitted to Nano Letter
C_8H_8: a density functional theory study of molecular geometries introducing the localised bond density
In this paper we use density functional theory with all the common exchange-correlation functionals to investigate the structures of three isomers of C_8H_8 found in F. A. Cotton's text, barrelene, cyclooctatetraene, tetramethylenecyclobutane and also ethane and ethene. All calculations were performed with TZ2P basis sets and large quadrature. The results are compared with experiment and those obtained with Hartree–Fock theory. Delocalisation in the three molecules is discussed. A localised bond density in introduced to explain the transferability of the trends in the predictions of the functionals between different molecules. Three-parameter adiabatic connection functionals are examined and their usefulness in geometry prediction questioned. Finally a physical picture of the correlation as modelled by density functional theory is presented and used to explain trends in the overestimation or underestimation of bond lengths
Exploring the magnetic properties of the largest single molecule magnets
The giant {Mn₇₀} and {Mn₈₄} wheels are the largest nuclearity single-molecule magnets synthesized to date, and understanding their magnetic properties poses a challenge to theory. Starting from first-principles calculations, we explore the magnetic properties and excitations in these wheels using effective spin Hamiltonians. We find that the unusual geometry of the superexchange pathways leads to weakly coupled {Mn₇} subunits carrying an effective S = 2 spin. The spectrum exhibits a hierarchy of energy scales and massive degeneracies, with the lowest-energy excitations arising from Heisenberg-ring-like excitations of the {Mn₇} subunits around the wheel. We further describe how weak longer-range couplings can select the precise spin ground-state of the Mn wheels out of the nearly degenerate ground-state band
A projected approximation to strongly contracted N-electron valence perturbation theory for DMRG wavefunctions
A novel approach to strongly contracted N-electron valence perturbation theory (SC-NEVPT2) as a means of describing dynamic electron correlation for quantum chemical density matrix renormalization group (DMRG) calculations is presented. In this approach the strongly contracted perturber functions are projected onto a renormalized Hilbert space. Compared to a straightforward implementation of SC-NEVPT2 with DMRG wavefunctions, the computational scaling and storage requirements are reduced. This favorable scaling opens up the possibility of calculations with larger active spaces. A specially designed renormalization scheme ensures that both the electronic ground state and the perturber functions are well represented in the renormalized Hilbert space. Test calculations on the N_2 and [Cu_2O_2(en)_2]^(2+) demonstrate some key properties of the method and indicate its capabilities
Targeted Excited State Algorithms
To overcome the limitations of the traditional state-averaging approaches in
excited state calculations, where one solves for and represents all states
between the ground state and excited state of interest, we have investigated a
number of new excited state algorithms. Building on the work of van der Vorst
and Sleijpen (SIAM J. Matrix Anal. Appl., 17, 401 (1996)), we have implemented
Harmonic Davidson and State-Averaged Harmonic Davidson algorithms within the
context of the Density Matrix Renormalization Group (DMRG). We have assessed
their accuracy and stability of convergence in complete active space DMRG
calculations on the low-lying excited states in the acenes ranging from
naphthalene to pentacene. We find that both algorithms offer increased accuracy
over the traditional State-Averaged Davidson approach, and in particular, the
State-Averaged Harmonic Davidson algorithm offers an optimal combination of
accuracy and stability in convergence
A practical guide to density matrix embedding theory in quantum chemistry
Density matrix embedding theory (DMET) provides a theoretical framework to
treat finite fragments in the presence of a surrounding molecular or bulk
environment, even when there is significant correlation or entanglement between
the two. In this work, we give a practically oriented and explicit description
of the numerical and theoretical formulation of DMET. We also describe in
detail how to perform self-consistent DMET optimizations. We explore different
embedding strategies with and without a self-consistency condition in hydrogen
rings, beryllium rings, and a sample S2 reaction. The source code
for the calculations in this work can be obtained from
\url{https://github.com/sebwouters/qc-dmet}.Comment: 41 pages, 10 figure
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