1,188 research outputs found
Generalized self-energy embedding theory
Ab initio quantum chemistry calculations for systems with large active spaces
are notoriously difficult and cannot be successfully tackled by standard
methods. In this letter, we generalize a Green's function QM/QM embedding
method called self-energy embedding theory (SEET) that has the potential to be
successfully employed to treat large active spaces. In generalized SEET, active
orbitals are grouped into intersecting groups of few orbitals allowing us to
perform multiple parallel calculations yielding results comparable to the full
active space treatment. We examine generalized SEET on a series of examples and
discuss a hierarchy of systematically improvable approximations
Reaching high accuracy for energetic properties at second-order perturbation cost by merging self-consistency and spin-opposite scaling
Quantum chemical methods dealing with challenging systems while retaining low
computational costs have attracted attention. In particular, many efforts have
been devoted to developing new methods based on the second-order perturbation
that may be the simplest correlated method beyond Hartree-Fock. We have
recently developed a self-consistent perturbation theory named one-body
M{\o}ller-Plesset second-order perturbation theory (OBMP2) and shown that it
can resolve issues caused by the non-iterative nature of standard perturbation
theory. In the present work, we extend the method by introducing the
spin-opposite scaling to the double-excitation amplitudes, resulting in the
O2BMP2 method. We assess the O2BMP2 performance on the triple-bond N2
dissociation, singlet-triplet gaps, and ionization potentials. O2BMP2 performs
much better than standard MP2 and reaches the accuracy of coupled-cluster
methods in all cases considered in this work.Comment: 22 pages, 9 figures, 2 table
Tracking excited states in wave function optimization using density matrices and variational principles
We present a method for finding individual excited states' energy stationary
points in complete active space self-consistent field theory that is compatible
with standard optimization methods and highly effective at overcoming
difficulties due to root flipping and near-degeneracies. Inspired by both the
maximum overlap method and recent progress in excited state variational
principles, our approach combines these ideas in order to track individual
excited states throughout the orbital optimization process. In a series of
tests involving root flipping, near-degeneracies, charge transfers, and double
excitations, we show that this approach is more effective for state-specific
optimization than either the naive selection of roots based on energy ordering
or a more direct generalization of the maximum overlap method. Furthermore, we
provide evidence that this state-specific approach improves the performance of
complete active space perturbation theory. With a simple implementation, a low
cost, and compatibility with large active space methods, the approach is
designed to be useful in a wide range of excited state investigations.Comment: 13 pages, submitted to JCT
Improving excited state potential energy surfaces via optimal orbital shapes
We demonstrate that, rather than resorting to high-cost dynamic correlation
methods, qualitative failures in excited-state potential energy surface
predictions can often be remedied at no additional cost by ensuring that
optimal molecular orbitals are used for each individual excited state. This
approach also avoids the weighting choices required by state-averaging and
dynamic weighting and obviates their need for expensive wave function response
calculations when relaxing excited state geometries. Although multi-state
approaches are of course preferred near conical intersections, other features
of excited-state potential energy surfaces can benefit significantly from our
single state approach. In three different systems, including a double bond
dissociation, a biologically relevant amino hydrogen dissociation, and an
amino-to-ring intramolecular charge transfer, we show that state-specific
orbitals offer qualitative improvements over the state-averaged status quo.Comment: 6 pages, 6 figures, 1 tabl
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