Multireference perturbation CI I. Extrapolation procedures with CAS or selected zero-order spaces |
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Authors: | Celestino Angeli Renzo Cimiraglia Maurizio Persico Alessandro Toniolo |
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Institution: | (1) Scuola Normale Superiore di Pisa, p.za Cavalieri 7, I-56100 Pisa, Italy, IT;(2) Dipartimento di Chimica, Università di Ferrara, Via Borsari 46, I-44100 Ferrara, Italy, IT;(3) Dipartimento di Chimica e Chim. Ind., Università di Pisa, v. Risorgimento 35, I-56126 Pisa, Italy e-mail: mau@hermes.dcci.unipi.it, IT |
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Abstract: | We discuss the “three class” approximation to full multireference perturbation CI, which greatly reduces the computational
effort by restricting the summation of diagrams to determinants belonging to a subspace of the zero-order space. In the framework
of the CIPSI algorithm, we propose a new extrapolation procedure allowing recovery of the full “two class” results. The new
procedure is applied to complete active spaces (CAS) and to individually selected zero-order spaces. Comparison with a full
two class calculation on a CAS shows a reduction of computer time of one or two orders of magnitude in the tests presented
here, with an accuracy in the order of 0.1 kcal/mol. Our procedure can thus compete with the CASPT2 algorithm, specifically
conceived to deal with CAS. In the case of selected zero-order spaces, the speed-up is less dramatic but the method still
retains its advantages.
Received: 12 June 1997 / Accepted: 31 July 1997 |
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Keywords: | : Multireference CI Perturbation CI CIPSI |
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