Why configuration interaction

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Selected configuration interaction SCI methods are currently enjoying a resurgence due to several recent developments which improve either the overall computational efficiency or the compactness of the resulting SCI vector. These recent advances have made it possible to get full CI FCI quality results for much larger orbital active spaces compared to conventional approaches. However, due to the starting assumption that the FCI vector has only a small number of significant Slater determinants, SCI becomes intractable for systems with strong correlation.

This paper introduces a method for developing SCI algorithms in a way which exploits local molecular structure to significantly reduce the number of SCI variables. The proposed method is defined by first grouping the orbitals into clusters over which we can define many-particle cluster states. We then directly perform the SCI algorithm in a basis of tensor products of cluster states instead of Slater determinants. While the approach is general for arbitrarily defined cluster states, we find significantly improved performance by defining cluster states through a Tucker decomposition of the global and sparse SCI vector.

These numerical results show that TPSCI can be used to significantly reduce the number of SCI variables in the variational space, thus paving a path for extending these deterministic and variational SCI approaches to a wider range of physical systems.

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Theory Comput. More by Vibin Abraham. More by Nicholas J. Cite this: J. Article Views Altmetric -. Citations It is, however, computationally extremely expensive and so generally the basis set is limited to a finite size. For example, the ground state of beryllium would be represented by the Hartree-Fock SCF method as a linear combination of the 1s and 2s orbitals. In configuration interaction, contributions from excited states involving the 1s and 3s orbitals, for example, would be included in a linear variation function for the ground state.

Configuration interaction is useful for calculating excited states of molecules, where the Hartree-Fock method invariably fails. For a much more detailed approach, the configuration interaction approach is described in a paper by G. Yan, et al. Figure 8. In conventional CI, the SCF wavefunction is used as a starting point called the reference function for obtaining the configuration state functions.