ISSN:
0020-7608
Keywords:
Computational Chemistry and Molecular Modeling
;
Atomic, Molecular and Optical Physics
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The energy at sixth-order Møller-Plesset (MP6) perturbation theory is given and dissected into 36 size-consistent energy contributions resulting from single (S), double (D), triple (T), quadruple (Q), pentuple (P), and hextuple (H) excitations. It is shown that MP6 is an O(N9) method, but less costly approximations to MP6 are possible. MP6 is used to analyze and compare coupled cluster (CC) and quadratic configuration interaction (QCI) methods, namely CCD, CCSD, CCSD(T), CCSD(TQ), CCSDT, CCSDT(Q), CCSDT(QQ), QCISD, QCISD(T), and QCISD(TQ). For larger molecules and molecules with distinct T contributions, CCSD is significantly better than QCISD because CCSD covers a relatively large number of T contributions and in particular T,T coupling effects at sixth order. Differences between the two methods become larger at higher orders of perturbation theory. If T and Q excitations are included in QCISD and CCSD in a noniterative way - thus leading to QCISD(T), CCSD(T), QCISD(TQ), and CCSD(TQ) - then differences between QCI and CC decrease. Hence, if a given molecular problem depends on the inclusion of T effects, improved calculational results will be obtained in the following order: MP4(SDTQ) 〈 QCISD(T) 〈 CCSD(T) 〈 QCISD(TQ), CCSD(TQ) 〈 CCSDT. None of the methods investigated is correct in sixth order. Only if CCSDT is extended to CCSDT(QPH), which is also an O(N9) method, are all MP6 energy contributions then covered.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/qua.560400809
Permalink
