1,032 research outputs found
Benchmark of dynamic electron correlation models for seniority-zero wavefunctions and their application to thermochemistry
Wavefunctions restricted to electron-pair states are promising models to
describe static/nondynamic electron correlation effects encountered, for
instance, in bond-dissociation processes and transition-metal and actinide
chemistry. To reach spectroscopic accuracy, however, the missing dynamic
electron correlation effects that cannot be described by electron-pair states
need to be included \textit{a posteriori}. In this article, we extend the
previously presented perturbation theory models with an Antisymmetric Product
of 1-reference orbital Geminal (AP1roG) reference function that allow us to
describe both static/nondynamic and dynamic electron correlation effects.
Specifically, our perturbation theory models combine a diagonal and
off-diagonal zero-order Hamiltonian, a single-reference and multi-reference
dual state, and different excitation operators used to construct the projection
manifold. We benchmark all proposed models as well as an \textit{a posteriori}
linearized coupled cluster correction on top of AP1roG against CR-CCSD(T)
reference data for reaction energies of several closed-shell molecules that are
extrapolated to the basis set limit. Moreover, we test the performance of our
new methods for multiple bond breaking processes in the N, C, and BN
dimers against MRCI-SD and MRCI-SD+Q reference data. Our numerical results
indicate that the best performance is obtained from a linearized coupled
cluster correction as well as second-order perturbation theory corrections
employing a diagonal and off-diagonal zero-order Hamiltonian and a
single-determinant dual state. These dynamic corrections on top of AP1roG allow
us to reliably model molecular systems dominated by static/nondynamic as well
as dynamic electron correlation.Comment: 15 pages, 2 figure
Linearized Coupled Cluster Correction on the Antisymmetric Product of 1 reference orbital Geminals
We present a Linearized Coupled Cluster (LCC) correction based on an
Antisymmetric Product of 1 reference orbital Geminals (AP1roG) reference state.
In our LCC ansatz, the cluster operator is restricted to double and to single
and double excitations as in standard single-reference CC theory. The
performance of the AP1roG-LCC models is tested for the dissociation of diatomic
molecules (C and F), spectroscopic constants of the uranyl cation
(UO), and the symmetric dissociation of the H hydrogen chain.
Our study indicates that an LCC correction based on an AP1roG reference
function is more robust and reliable than corrections based on perturbation
theory, yielding spectroscopic constants that are in very good agreement with
theoretical reference data.Comment: 9 pages, 4 figure
Optimized Unrestricted Kohn-Sham Potentials from Ab Initio Spin Densities
The reconstruction of the exchange-correlation potential from accurate ab
initio electron densities can provide insights into the limitations of the
currently available approximate functionals and provide guidance for devising
improved approximations for density-functional theory (DFT). For open-shell
systems, the spin density is introduced as an additional fundamental variable
in Spin-DFT. Here, we consider the reconstruction of the corresponding
unrestricted Kohn-Sham potentials from accurate ab initio spin densities. In
particular, we investigate whether it is possible to reconstruct the spin
exchange-correlation potential, which determines the spin density in
spin-unrestricted Kohn-Sham-DFT, despite the numerical difficulties inherent to
the optimization of potentials with finite orbital basis sets. We find that the
recently developed scheme for unambiguously singling out an optimal optimized
potential [J. Chem. Phys. 135, 244102 (2011)] can provide such spin potentials
accurately. This is demonstrated for two test cases, the lithium atom and the
dioxygen molecule, and target (spin) densities from Full-CI and CASSCF
calculations, respectively
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