Nonorthogonal approaches to electronic structure methods have recently received renewed attention, with the hope that new forms of nonorthogonal wavefunction Ansätze may circumvent the computational bottleneck of orthogonal-based methods. The basis in which nonorthogonal configuration interaction is performed defines the compactness of the wavefunction description and hence the efficiency of the method. Within a molecular orbital approach, nonorthogonal configuration interaction is defined by a “different orbitals for different configurations” picture, with different methods being defined by their choice of determinant basis functions. However, identification of a suitable determinant basis is complicated, in practice, by (i) exponential scaling of the determinant space from which a suitable basis must be extracted, (ii) possible linear dependencies in the determinant basis, and (iii) inconsistent behavior in the determinant basis, such as disappearing or coalescing solutions, as a result of external perturbations, such as geometry change. An approach that avoids the aforementioned issues is to allow for basis determinant optimization starting from an arbitrarily constructed initial determinant set. In this work, we derive the equations required for performing such an optimization, extending previous work by accounting for changes in the orthogonality level (defined as the dimension of the orbital overlap kernel between two determinants) as a result of orbital perturbations. The performance of the resulting wavefunction for studying avoided crossings and conical intersections where strong correlation plays an important role is examined.
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28 June 2021
Research Article|
June 22 2021
Orbital optimization in nonorthogonal multiconfigurational self-consistent field applied to the study of conical intersections and avoided crossings
Andrew D. Mahler;
Andrew D. Mahler
Department of Chemistry, University of Louisville
, 2320 South Brook Street, Louisville, Kentucky 40292, USA
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Lee M. Thompson
Lee M. Thompson
a)
Department of Chemistry, University of Louisville
, 2320 South Brook Street, Louisville, Kentucky 40292, USA
a)Author to whom correspondence should be addressed: lee.thompson.1@louisville.edu
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a)Author to whom correspondence should be addressed: lee.thompson.1@louisville.edu
J. Chem. Phys. 154, 244101 (2021)
Article history
Received:
April 09 2021
Accepted:
June 03 2021
Citation
Andrew D. Mahler, Lee M. Thompson; Orbital optimization in nonorthogonal multiconfigurational self-consistent field applied to the study of conical intersections and avoided crossings. J. Chem. Phys. 28 June 2021; 154 (24): 244101. https://doi.org/10.1063/5.0053615
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