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CISDTQ

CISDTQ CI including single through quadruple electronic excitations... [Pg.550]

One interesting development that took place during the MRCI studies was the announcement by Grev and co-workers that their CISDTQ results were incorrect [90]. Using correct CISDTQ values to revise their adjustments to the CISD+Q results they obtained an estimate of 4+4 kcal/mol for the separation. This was now consistent with the assertion of Martin and co-workers that it was not possible to decide whether the minimum energy structure was cyclic or linear. [Pg.385]

Unfortunately, this harmonic frequency is also the only one for which no experimental value has been obtained, a fact that is hardly surprising given that the relevant transitions are essentially forbidden. However, comparison with the CISDTQ and CCSD(T) values given in Table 9 leads one to infer that the OBS-GMCSC value should not be in error by more than a few cm"1, or about 0.2%. The error on Re similarly appears to be of the order of 0.2%, or 0.003 A at most. As already noted, the only experimental value of the bond length refers to Rq (1.19001 A [40]), and thus constitutes only an upper bound to Re. [Pg.308]

In the last 20 years, Cl calculations based on a single reference function have lost favor among practitioners. The principal shortcoming of these approaches is that they do not satisfy the property of size-consistency, which means that the Cl energy does not scale properly with the size of the system [112]. It is fairly easy to see why this is so. Consider two beryllium atoms, separated by a distance sufficiently large that the true physical interaction between the atoms vanishes. In a CISD description of this system, contributions to the wave function are excluded in which two electrons on each beryllium atom are in virtual orbitals, since these correspond to quadruply excited determinants and would require a method such as CISDQ or CISDTQ for their inclusion. However, the CISD wave function for a single beryllium atom contains all determinants with two electrons in virtual orbitals. Since Cl methods involve... [Pg.106]

A rapid increase in the importance of higher-than-pair clusters is clearly illustrated by the sequence of SR Cl results. Even the SR CISDTQ NPEs amount to 12.6 and 38 mhartree. This clearly indicates the role played by higher-than-4-body (both connected and disconnected) clusters as R —> oo. [Pg.16]

Thus the number of operations for a CISD procedure has a sixth power dependence on the total number of orbitals, while CISDTQ scales as the tenth power. For a given system, the number of occupied orbitals is fixed, and the cost of increasing the basis set size scales as 0(n2+m) for CISD and CISDTQ, this scaling becomes 0(n4) and >(rj°), respectively. [Pg.166]

Few studies have employed configuration interaction with all singles, doubles, triples, and quadruples (CISDTQ) because the number of triple and quadruple substitutions grows very rapidly with the number of electrons and basis functions (cf. Table 3). CISDTQ results are most commonly reported in benchmark full Cl studies to indicate the fraction of the basis set correlation energy recovered by triples and quadruples.17,22,39,801234,241,264 Nevertheless, the CISDTQ method has occasionally been used for benchmarking in cases where the full Cl was not technically feasible, because the CISDTQ results are expected to be very close to full Cl for small molecules. For systems with eight electrons or less at their equilibrium geometries, the CISDTQ method recovers more... [Pg.247]


See other pages where CISDTQ is mentioned: [Pg.24]    [Pg.130]    [Pg.224]    [Pg.224]    [Pg.107]    [Pg.136]    [Pg.145]    [Pg.268]    [Pg.444]    [Pg.222]    [Pg.350]    [Pg.367]    [Pg.367]    [Pg.385]    [Pg.257]    [Pg.145]    [Pg.309]    [Pg.328]    [Pg.76]    [Pg.80]    [Pg.325]    [Pg.201]    [Pg.107]    [Pg.136]    [Pg.145]    [Pg.268]    [Pg.15]    [Pg.145]    [Pg.158]    [Pg.158]    [Pg.159]    [Pg.159]    [Pg.234]    [Pg.240]    [Pg.240]    [Pg.247]    [Pg.248]    [Pg.248]    [Pg.248]    [Pg.249]    [Pg.250]    [Pg.250]   
See also in sourсe #XX -- [ Pg.24 ]

See also in sourсe #XX -- [ Pg.84 , Pg.91 , Pg.96 ]

See also in sourсe #XX -- [ Pg.24 ]




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Configuration interaction CISDTQ

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