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Quantification of the Ionic Character of Multiconfigurational Wave Functions: The Q_a^t Diagnostic.

ABSTRACT: The complete active space self-consistent field (CASSCF) method is a cornerstone in modern excited-state quantum chemistry providing the starting point for most common multireference computations. However, CASSCF, when used with a minimal active space, can produce significant errors (>2 eV) even for the excitation energies of simple hydrocarbons if the states of interest possess ionic character. After illustrating this problem in some detail, we present a diagnostic for ionic character, denoted as Q _a^t, that is readily computed from the transition density. A set of 11 molecules is considered to study errors in vertical excitation energies. State-averaged CASSCF obtains a mean absolute error (MAE) of 0.87 eV for the 34 singlet states considered. We highlight a strong correlation between the obtained errors and the Q _a^t diagnostic, illustrating its power to predict problematic cases. Conversely, using multireference configuration interaction with single and double excitations and Pople's size extensivity correction (MR-CISD+P), excellent results are obtained with an MAE of 0.11 eV. Furthermore, correlations with the Q _a^t diagnostic disappear. In summary, we hope that the presented diagnostic will facilitate reliable and user-friendly multireference computations on conjugated organic molecules.

SUBMITTER: do Monte SA

PROVIDER: S-EPMC10683019 | biostudies-literature | 2023 Nov

REPOSITORIES: biostudies-literature

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Quantification of the Ionic Character of Multiconfigurational Wave Functions: The Qat Diagnostic.

do Monte Silmar A SA Spada Rene F K RFK Alves Rodolpho L R RLR Belcher Lachlan L Shepard Ron R Lischka Hans H Plasser Felix F

The journal of physical chemistry. A 20231018 46

The complete active space self-consistent field (CASSCF) method is a cornerstone in modern excited-state quantum chemistry providing the starting point for most common multireference computations. However, CASSCF, when used with a minimal active space, can produce significant errors (>2 eV) even for the excitation energies of simple hydrocarbons if the states of interest possess ionic character. After illustrating this problem in some detail, we present a diagnostic for ionic character, denoted ...[more]

PMID: 37851528

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Quantification of the Ionic Character of Multiconfigurational Wave Functions: The Q_a^t Diagnostic.

Publications

Quantification of the Ionic Character of Multiconfigurational Wave Functions: The <i>Q</i><sub>a</sub><sup>t</sup> Diagnostic.

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