Sixth-order many-body perturbation theory. III. Correlation energies of size-extensive MP6 methods
✍ Scribed by Zhi He; Dieter Cremer
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 943 KB
- Volume
- 59
- Category
- Article
- ISSN
- 0020-7608
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✦ Synopsis
A comparison of sixth-order Mdler-Plesset perturbation energies (MP6) with the corresponding full configuration interaction (FCI) energies shows that in the case of equilibrium geometries MP6 values differ by just 1.7 mhartree. MP6 correlation energies turn out to be important for systems with oscillatory convergence behavior as well as for systems with considerable multireference character. Contributions from pentuple ( P ) and hextuple ( H ) excitations are mostly positive and smaller than 1 mhartree in the cases investigated. Initial oscillations in the convergence behavior of a MPn series result from positive fifth-order T energy contributions to the correlation energy. In these cases, MP6 correlation energies are relatively large (T contributions dominate the total correlation energy) and absolutely necessary when estimating the convergence limit of the MPn series. MP6 is an O ( M 9 ) method and, therefore, can only be used for relatively small electron systems. More economic alternatives are given by the approximated MP6 methods MP6(M8) and MP6(M7), which involve O ( M 8 ) and O(M 7, operations, respectively. According to calculated absolute and relative energies, MP6(M7) is an attractive alternative to full MP6 because it offers reasonable sixth-order results for cost comparable to those of MP4. 0 1996 John Wiley & Sons, Inc. spin-orbital two-electron integral formulas [ 21. First applications of the new perturbation theory method have revealed that MP6 correlation energies come close to full configuration interaction (FCI) correlation energies obtained with the Same basis set at the Same geometry. Comparison with FCI energies is a standard procedure to assess the accuracy of results obtained with a new correlation method. In bation theory in terms of cluster operators [I] and *To whom correspondence should be addressed.