A non-linear approach to configuration interaction: The low-rank CI method (LR CI)
✍ Scribed by Jeppe Olsen; Per-åke Malmqvist; Björn O. Roos; Roland Lindh; Per-Olof Widmark
- Publisher
- Elsevier Science
- Year
- 1987
- Tongue
- English
- Weight
- 894 KB
- Volume
- 133
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
An alternative to the linear CI expansion is presented, which is based on the spectral resolution of the double excitation CI coeffkients arranged as a symmetric matrix. The resulting energy expression is a quartic function of the new variables, where the double excitation operator is given as fz = Xyw,( &)2 , where 2, is a single excitation operator: 2, = I&d:. I&,. Since M is a small number, the number of variables is reduced considerably compared to normal SDCI, with only little loss in accuracy. A program for non-linear CI calculations has been written and is presented with results for Hz0 and N2. The method can easily be extended to include orbital optimization and cluster terms in the wavefunction, still yielding a fully variational approximation to the Schrodinger equation.