18 research outputs found
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Non-orthogonal configuration interaction with single substitutions for core-excited states: An extension to doublet radicals
In this paper, we present an open-shell extension of the non-orthogonal configuration interaction singles (NOCIS) method for the calculation of core-excited states, intended for peak assignment in XAS spectra of doublet radicals. This extension requires the consideration of additional configurations due to the singly occupied open-shell orbital, and the addition of essential orbital relaxation effects is found to provide a significant improvement on standard CIS, while maintaining the desirable properties of spin purity, variationality, and size consistency. We apply this method to the calculation of core excitations for several open-shell molecules and demonstrate that it performs competitively with other available methods, despite a lack of dynamic correlation. In particular, relative to CVS-ADC(2)-x, RMS error is reduced by a factor of 6 over usual orthogonal CIS and is comparable to time-dependent density functional theory with the best short-range corrected functionals
Accurate prediction of core-level spectra of radicals at density functional theory cost via square gradient minimization and recoupling of mixed configurations
State-specific orbital optimized approaches are more accurate at predicting
core-level spectra than traditional linear-response protocols, but their
utility had been restricted on account of the risk of `variational collapse'
down to the ground state. We employ the recently developed square gradient
minimization (SGM, J. Chem. Theory Comput. 16, 1699-1710, 2020) algorithm to
reliably avoid variational collapse and study the effectiveness of orbital
optimized density functional theory (DFT) at predicting second period element
1s core-level spectra of open-shell systems. Several density functionals
(including SCAN, B3LYP and B97X-D3) are found to predict excitation
energies from the core to singly occupied levels to high accuracy ( eV
RMS error), against available experimental data. Higher excited states are
however more challenging by virtue of being intrinsically multiconfigurational.
We thus present a CI inspired route to self-consistently recouple single
determinant mixed configurations obtained from DFT, in order to obtain
approximate doublet states. This recoupling scheme is used to predict the C
K-edge spectra of the allyl radical, the O K-edge spectra of CO and the N
K-edge of NO to high accuracy relative to experiment, indicating
substantial promise in using this approach for computation of core-level
spectra for doublet species (vs more traditional time dependent DFT, EOM-CCSD
or using unrecoupled mixed configurations). We also present general guidelines
for computing core-excited states from orbital optimized DFT.Comment: Added more dat
Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package
This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design
Non-orthogonal configuration interaction with single substitutions for the calculation of core-excited states
Non-Orthogonal Configuration Interaction with Single Substitutions for Core-Excited States: An Extension to Doublet Radicals.
Generalized single excitation configuration interaction: an investigation into the impact of the inclusion of non-orthogonality on the calculation of core-excited states
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Non-orthogonal configuration interaction with single substitutions for the calculation of core-excited states
In this paper, we present the non-orthogonal configuration interaction singles (NOCIS) method for calculating core-excited states of closed-shell molecules. NOCIS is a black-box variant of NOCI, which uses A different core-ionized determinants for a molecule with A atoms of a given element to form single substitutions. NOCIS is a variational, spin-pure, size-consistent ab initio method that dramatically improves on standard CIS by capturing essential orbital relaxation effects, in addition to essential configuration interaction. We apply it to the calculation of core-excitations for several smaller molecules and demonstrate that it performs competitively with other Hartree-Fock and DFT-based methods. We also benchmark it in several basis sets
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Erratum: "Non-orthogonal configuration interaction with single substitutions for the calculation of core-excited states" [J. Chem. Phys. 149, 044116 (2018)].
In our recent publication,1 we incorrectly stated some of the CIS and TDDFT k-edge excitations. Specifically the errors were in the CIS k-edge for all molecules except C2N2 and C2H6 and the TDDFT k-edge for only C2H2, N2, CO2 O, F2, and C2H2, all of which are shown in Table II of the original paper. A corrected version of that table can be found below. This changes our results slightly in that the RMSEs of the CIS and the TDDFT results are slightly reduced, but it does not change the primary conclusion that NOCIS is a promising method for calculating core excitations though it lacks dynamical correlation. (Table Presented)