CCSD calculations applications

The S-T splitting is obtained as a difference of the two separate individual energy calculations for the singlet and triplet. Those individual singlet and triplet calculations do not necessarily have to be by the same method, and in fact, the majority of the results in Table 1 use composite methods in which the singlet and triplet are treated by two different methods (primarily the singlet as a MR state while the triplet is SR). RMR indicates the reduced multi-reference approach of Li and Paldus [58]. The TD-CCSD of Baikova and Bartlett [54], an early SU-CCSD application, provides two roots simultaneously at each geometry. This SU-CCSD calculation introduced GVB CCSD as a two-determinant reference. The MR-BW is a state-specific MR... 

Although there is no strict relationship between the basis sets developed for, and used in, conventional ah initio calculations and those applicable in DFT, the basis sets employed in molecular DFT calculations are usually the same or highly similar to those. For most practical purposes, a standard valence double-zeta plus polarization basis set (e.g. the Pople basis set 6-31G(d,p) [29] and similar) provides sufficiently accurate geometries and energetics when employed in combination with one of the more accurate functionals (B3LYP, PBEO, PW91). A somewhat sweeping statement is that the accuracy usually lies mid-way between that of M P2 and that of the CCSD(T) or G2 conventional wave-function methods. 

Our study has been restricted to molecules containing only first-row atoms and with wavefunctions dominated by one determinant. Molecules such as 03 are less accurately described, with an error of about 10 kJ/mol at the CCSD(T) level of theory. For such multiconfigurational systems, more elaborate treatments are necessary and no programs are yet available for routine applications. As we go down the periodic table, relativistic effects become more important and the electronic structures more complicated. Therefore, for such systems it is presently not possible to calculate thermochemical data to the same accuracy as for closed-shell molecules containing first-row atoms. Nevertheless, systems with wave-functions dominated by single determinant are by far the most abundant and it is promising that the accuracy of a few kJ/mol is obtainable for them. 

CCSDTQ (CC singles, doubles, triples, and quadruples) (75-75), CCSDTQP (CC singles, doubles, triples, quadruples, and pentuples) (7P), etc. approaches are far too expensive for routine applications. For example, the full CCSDTQ method requires iterative steps that scale as ( g(/i )is the number of occupied (unoccupied) orbitals in the molecular orbital basis). This scaling restricts the applicability of the CCSDTQ approach to very small systems, consisting of 2 - 3 light atoms described by small basis sets. For comparison, CCSD(T) is an nln procedure in the iterative CCSD steps and an nl procedure in the non-iterative part related to the calculation of the triples (T) correction. In consequence, it is nowadays possible to perform the CCSD(T) calculations for systems with 10-20 atoms. The application of the local correlation formalism (80-82) enabled SchOtz and Werner to extend the applicability of the CCSD(T) approach to systems with 100 atoms (53, 83, 84). 

Recent developments in computational chemistry have established the exact structure of carbocations by combining computational and experimental results.78,79 Furthermore, accurate 1H and 13C NMR chemical shifts of carbocations and other organic molecules can be calculated with the application of recent coupled cluster methods, such as GIAO-CCSD(T).80... 

The resulting SAPT(DFT) potential energy curves turn out to be very accurate in the wide range of intermolecular separations. For the benzene dimer225,228 the results are very close to those of the much more expensive CCSD(T) treatment. For systems of the size of the benzene dimer and for the triple-zeta quality basis sets, a SAPT(DFT) calculation actually takes less time than a conventional supermolecular DFT calculation. Due to the favorable computational scaling the SAPT(DFT) approach is applicable to much larger molecules than any method used thus far for a reliable calculation of dispersion-dominated interaction potential. 

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