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Download file# NR2 and P3+: Accurate, Efficient Electron-Propagator Methods for Calculating Valence, Vertical Ionization Energies of Closed-Shell Molecules

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posted on 2015-08-20, 00:00 authored by H. H. Corzo, Annia Galano, O. Dolgounitcheva, V. G. Zakrzewski, J. V. OrtizTwo accurate and computationally
efficient electron-propagator
(EP) methods for calculating the valence, vertical ionization energies
(VIEs) of closed–shell molecules have been identified through
comparisons with related approximations. VIEs of a representative
set of closed-shell molecules were calculated with EP methods using
10 basis sets. The most easily executed method, the diagonal, second-order
(D2) EP approximation, produces results that steadily rise as basis
sets are improved toward values based on extrapolated coupled-cluster
singles and doubles plus perturbative triples calculations, but its
mean errors remain unacceptably large. The outer valence Green function,
partial third-order and renormalized partial third-order methods (P3+),
which employ the diagonal self-energy approximation, produce markedly
better results but have a greater tendency to overestimate VIEs with
larger basis sets. The best combination of accuracy and efficiency
with a diagonal self-energy matrix is the P3+ approximation, which
exhibits the best trends with respect to basis-set saturation. Several
renormalized methods with more flexible nondiagonal self-energies
also have been examined: the two-particle, one-hole Tamm–Dancoff
approximation (2ph-TDA), the third-order algebraic diagrammatic construction
or ADC(3), the renormalized third-order (3+) method, and the nondiagonal
second-order renormalized (NR2) approximation. Like D2, 2ph-TDA produces
steady improvements with basis set augmentation, but its average errors
are too large. Errors obtained with 3+ and ADC(3) are smaller on average
than those of 2ph-TDA. These methods also have a greater tendency
to overestimate VIEs with larger basis sets. The smallest average
errors occur for the NR2 approximation; these errors decrease steadily
with basis augmentations. As basis sets approach saturation, NR2 becomes
the most accurate and efficient method with a nondiagonal self-energy.