Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A view on coupled cluster perturbation theory using a bivariational
  Lagrangian formulation   We consider two distinct coupled cluster (CC) perturbation series that both
expand the difference between the energies of the CCSD (CC with single and
double excitations) and CCSDT (CC with single, double, and triple excitations)
models in orders of the M{\o}ller-Plesset fluctuation potential. We initially
introduce the E-CCSD(T-$n$) series, in which the CCSD amplitude equations are
satisfied at the expansion point, and compare it to the recently developed
CCSD(T-$n$) series [J. Chem. Phys. 140, 064108 (2014)], in which not only the
CCSD amplitude, but also the CCSD multiplier equations are satisfied at the
expansion point. The computational scaling is similar for the two series, and
both are term-wise size extensive with a formal convergence towards the CCSDT
target energy. However, the two series are different, and the CCSD(T-$n$)
series is found to exhibit a more rapid convergence up through the series,
which we trace back to the fact that more information at the expansion point is
utilized than for the E-CCSD(T-$n$) series. The present analysis can be
generalized to any perturbation expansion representing the difference between a
parent CC model and a higher-level target CC model. In general, we demonstrate
that, whenever the parent parameters depend upon the perturbation operator, a
perturbation expansion of the CC energy (where only parent amplitudes are used)
differs from a perturbation expansion of the CC Lagrangian (where both parent
amplitudes and parent multipliers are used). For the latter case, the
bivariational Lagrangian formulation becomes more than a convenient
mathematical tool, since it facilitates a different and faster convergent
perturbation series than the simpler energy-based expansion.

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The categories of this paper are: supr-con
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The categories of this paper are: cmp-lg
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The categories of this paper are: adap-org
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The categories of this paper are: dg-ga
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The categories of this paper are: mtrl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: cs
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The categories of this paper are: patt-sol
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The categories of this paper are: q-fin
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The categories of this paper are: chem-ph
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The categories of this paper are: math-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: astro-ph