Single excitation configuration interactions approach

Combination of DFT and single excitation configuration interaction approach (DFT/ SCI) Theoretical calculation of CD spectrum, i.e., excitation energies AE and rotatory strengths R 1999TA3483... 

Two other types of basis set that have been used successfully in hfs calculations are Chipman s contracted [3s,2p] bases, and basis sets based on Slater type orbitals (STOs). The former of these is mainly used in single excitation configuration interaction (CIS) calculations, and are based on a very fortuitous cancellation of errors between method and basis set. The performance of the CIS/[3s,2p] approach lies within 20-25% of experiment. One should recall, though, that once we go to larger molecular systems, the CIS method becomes computationally very demanding, STOs have mainly been used in semiempirical INDO hfcc calculations (STO-SG) and in the density functional theory (DFT) studies of Ishii and Shimitzu (STO-6G). The number of hfcc studies using these basis sets at the ab initio or DFT levels is however to date very limited. 

The second step of the calculation involves the treatment of dynamic correlation effects, which can be approached by many-body perturbation theory (62) or configuration interaction (63). Multireference coupled-cluster techniques have been developed (64—66) but they are computationally far more demanding and still not established as standard methods. At this point, we will only focus on configuration interaction approaches. What is done in these approaches is to regard the entire zeroth-order wavefunc-tion Tj) or its constituent parts double excitations relative to these reference functions. This produces a set of excited CSFs ( Q) that are used as expansion space for the configuration interaction (Cl) procedure. The resulting wavefunction may be written as... 

In Tables 2 and 3, triplet doubly excited energies of 2s ns (n = 3,4,. .10) states and 3s ns (n = 4, 5,11) states of He, computed at the CSCF level, are presented. Calculations of Ref. [46] were restricted to only singly excited states. Therefore, we compare our CSCF calculations with accurate theoretical calculations based on a configuration interaction approach with the explicitly correlated HyUeraas basis set functions [48]. One can see that the accuracy of the CSCF calculations is improved when n increases. This observation is in agreement with Ref. [46]. whose authors pointed out that In those states where n 1, the electrons are spatially well separated and one might anticipate intuitively that they will be weakly correlated and that the Hartree-Fock method, which neglects such effects, may be an excellent approximation. ... 

As has been shown by, e.g., Chipman, the main contributions to spin polarization lie in the single excitations. Hence, the use of a configuration interaction approach based on the ROHF wave function can be expected to provide significantly improved hfcc. Several such schemes are available, the most common of which are based on the inclusion of single or single and double excitations (CIS, CISD). 

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