2,571 research outputs found
Orbital optimization in the perfect pairing hierarchy. Applications to full-valence calculations on linear polyacenes
We describe the implementation of orbital optimization for the models in the
perfect pairing hierarchy [Lehtola et al, J. Chem. Phys. 145, 134110 (2016)].
Orbital optimization, which is generally necessary to obtain reliable results,
is pursued at perfect pairing (PP) and perfect quadruples (PQ) levels of theory
for applications on linear polyacenes, which are believed to exhibit strong
correlation in the {\pi} space. While local minima and {\sigma}-{\pi} symmetry
breaking solutions were found for PP orbitals, no such problems were
encountered for PQ orbitals. The PQ orbitals are used for single-point
calculations at PP, PQ and perfect hextuples (PH) levels of theory, both only
in the {\pi} subspace, as well as in the full {\sigma}{\pi} valence space. It
is numerically demonstrated that the inclusion of single excitations is
necessary also when optimized orbitals are used. PH is found to yield good
agreement with previously published density matrix renormalization group (DMRG)
data in the {\pi} space, capturing over 95% of the correlation energy.
Full-valence calculations made possible by our novel, efficient code reveal
that strong correlations are weaker when larger bases or active spaces are
employed than in previous calculations. The largest full-valence PH
calculations presented correspond to a (192e,192o) problem.Comment: 19 pages, 4 figure
Second-Order Self-Consistent-Field Density-Matrix Renormalization Group
We present a matrix-product state (MPS)-based quadratically convergent
density-matrix renormalization group self-consistent-field (DMRG-SCF) approach.
Following a proposal by Werner and Knowles (JCP 82, 5053, (1985)), our DMRG-SCF
algorithm is based on a direct minimization of an energy expression which is
correct to second-order with respect to changes in the molecular orbital basis.
We exploit a simultaneous optimization of the MPS wave function and molecular
orbitals in order to achieve quadratic convergence. In contrast to previously
reported (augmented Hessian) Newton-Raphson and super-configuration-interaction
algorithms for DMRG-SCF, energy convergence beyond a quadratic scaling is
possible in our ansatz. Discarding the set of redundant active-active orbital
rotations, the DMRG-SCF energy converges typically within two to four cycles of
the self-consistent procedureComment: 40 pages, 5 figures, 3 table
Ground-state phase diagram of the three-band Hubbard model from density matrix embedding theory
We determine the ground-state phase diagram of the three-band Hubbard model across a range of model parameters using density matrix embedding theory. We study the atomic-scale nature of the antiferromagnetic (AFM) and superconducting (SC) orders, explicitly including the oxygen degrees of freedom. All parametrizations of the model display AFM and SC phases, but the decay of AFM order with doping is too slow compared to the experimental phase diagram, and further, coexistence of AFM and SC orders occurs in all parameter sets. The local magnetic moment localizes entirely at the copper sites. The magnetic phase diagram is particularly sensitive to Δ_(pd) and t_(pp), and existing estimates of the charge transfer gap Δ_(pd) appear too large in so-called minimal model parametrizations. The electron-doped side of the phase diagram is qualitatively distinct from the hole-doped side and we find an unusual two-peak structure in the SC in the full model parametrization. Examining the SC order at the atomic scale, within the larger scale d_(x²−y²)-wave SC pairing order between Cu-Cu and O-O, we also observe a local p_(x(y)) [or d_(xz(yz))] symmetry modulation of the pair density on the Cu-O bonds. Our work highlights some of the features that arise in a three-band versus one-band picture, the role of the oxygen degrees of freedom in new kinds of atomic-scale SC orders, and the necessity of re-evaluating current parametrizations of the three-band Hubbard model
Multiconfigurational Short-Range Density-Functional Theory for Open-Shell Systems
Many chemical systems cannot be described by quantum chemistry methods based
on a singlereference wave function. Accurate predictions of energetic and
spectroscopic properties require a delicate balance between describing the most
important configurations (static correlation) and obtaining dynamical
correlation efficiently. The former is most naturally done through a
multiconfigurational (MC) wave function, whereas the latter can be done by,
e.g., perturbation theory. We have employed a different strategy, namely, a
hybrid between multiconfigurational wave functions and density-functional
theory (DFT) based on range separation. The method is denoted by MC short-range
(sr) DFT and is more efficient than perturbative approaches as it capitalizes
on the efficient treatment of the (short-range) dynamical correlation by DFT
approximations. In turn, the method also improves DFT with standard
approximations through the ability of multiconfigurational wave functions to
recover large parts of the static correlation. Until now, our implementation
was restricted to closed-shell systems, and to lift this restriction, we
present here the generalization of MC-srDFT to open-shell cases. The additional
terms required to treat open-shell systems are derived and implemented in the
DALTON program. This new method for open-shell systems is illustrated on
dioxygen and [Fe(H2O)6]3+.Comment: 37 pages, 3 figures, 4 tables, 1 appendix and 79 references Changes
in v2: 1) Appendix B and reference 81 removed 2) Removed dublicated reference
and corrected reference 31. 3) Added spin-charge cross terms to GGA (Appendix
A). Code changed accordingly and GGA results recalculated. All GGA results
are revised -only small modifications observed. Conclusions are unchange
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