Correlation energies in open shell systems. Comparison of CEPA,PNO-CI and perturbation treatments based on the restricted Roothaan-Hartree-Fock formalism |
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| Authors: | Petr Čársky Ivan Hubač Volker Staemmler |
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| Institution: | (1) J. Heyrovsk y Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Máchova 7, 12138 Prague 2, Czechoslovakia;(2) Department of Biophysics, Faculty of Mathematics and Physics, Comenius University, Mlynská dolina, 81631 Bratislava, Czechoslovakia;(3) Lehrstuhl für Theoretische Chemie der Ruhr-Universität Bochum, D-4630 Bochum, Federal Republic of Germany |
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| Abstract: | A set of simple molecules in closed and open-shell ground states is treated by the three techniques mentioned in the title, using the same geometries and basis sets (DZ + P). It is found that for nearly all molecules treated in this study (exceptions are H2 and CH3) consistently about 98% of the CEPA valence shell correlation energy is obtained by third-order many-body Ray-leigh-Schrödinger perturbation theory (MB-RSPT). The CEPA and MB-RSPT results for reaction energies and barrier heights for some simple reactions differ by 0 to 30 kJ/mol, the CEPA results being in most cases closer to experiment than MB-RSPT, while CI results are much less reliable as long as CI is limited to singly and doubly substituted configurations only. |
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| Keywords: | Correlation energy of radicals Many-body perturbation theory CEPA |
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