Methods CI

NakatsujI H, Kuwano R, Merita H and Nakal H 1993 Dipped adcluster model and SAC-CI method applied to harpooning, chemical luminescence and electron emission in halogen chemisorption on alkali metal surface J. Mol. Catal. 82 211-28... 

The configuration interaction (CI) method in whieh the LCAO-MO eoeffieients are determined first (and independently) via either a single-eonfiguration SCF ealeulation or an MCSCF ealeulation using a small number of CSFs. The CI eoeffieients are subsequently determined by making the expeetation value < F H F >/< F I F >... 

In the CI method, one usually attempts to realize a high-level treatment of electron correlation. A set of orthonormal molecular orbitals are first obtained from an SCF or MCSCF calculation (usually involving a small to moderate list of CSFs). The FCAO-MO... 

The extended CIS method (XCIS) is a version of CIS for examining states that are doubly excited from the reference state. It does not include correlation and is thus similar in accuracy to CIS. 

Practical configuration interaction methods augment the Hartree-Fock by adding only a limited set of substitutions, truncating the Cl expansion at some level of substitution. For example, the CIS method adds single excitations to the Hartree-Fock determinant, CID adds double excitations, CISD adds singles and doubles, CISDT adds singles, doubles, and triples, and so on. 

On the theoretical hand, calculations have been performed as soon as in the 50ies [56,63] since formaldehyde represents the smallest member of the carbonyl series. References to early works are avalaible in the compilation by Davidson and McMurchie [64] and in references [56-58,63]. Of particular interest for a comprehensive assignment of the experimental transitions are the very fine and accurate calculations by Harding and Goddard using their GVB-CI method [60,65]. 

The calculations of the and c constants lead to a system of linear equations similar to that of the SCF-CI method, but with three more lines and columns corresponding to the coupling of the polynomial function with the electric field perturbation. The methodology and computational details have already been discussed (1) we stress two points the role of the dipolar factor, the nature and the number of the exeited states to inelude in the summation. 

Within the SCF-CI method a fixed set of molecular orbitals is used. This means that during the calculation (leading to slow convergence) the individual molecular orbitals remain unchanged. A method where the linear expansion coefficients and the LCAO coefficients are optimized simultaneously is the multi-configuration SCF (MCSCF). 

The second fixation reaction that has been the subject of theoretical study is the nickel(0)-catalyzed coupling reaction of C02 with acetylene (equation 1), which was theoretically investigated with the ab initio SD-CI method [25]. The theoretical calculations clearly showed that if the nickel(O) moiety was eliminated from the reaction system, the activation barrier increased very much, and that the C-C bond formation between C02 and acetylene was accelerated by the charge-transfer from the nickel(O) d orbital to the orbital resulting from the combination between n orbitals of acetylene and CO2. Actually, the HOMO contour map of Ni(PH3)(C02)(C2H2) clearly displays this orbital mixing in the transition state. 

The activation barrier and the reaction energy were evaluated with various computational methods, including MP4 (SDQ), SD-CI and CCD [34], The reaction was found to be exothermic by a value of about 40 kcal/mol. The activation barriers were found to be 9.9 kcal/mol at the MP4(SDQ) level, 4.2 at the SD-CI level and 4.3 at the CCD level. Apparently, the MP4(SDQ) method yields a much larger activation barrier than SD-CI and CCD methods. Since the CCD and SD-CI methods present a similar activation barrier, these values were taken as the correct ones. From these calculations, it was concluded that C02 is easily inserted into the Cu(I)-H bond with a moderate activation barrier. 

P.E.M. Siegbahn The Direct-CI Method., in G.H.F. Diercksen and S. Wilson (ed.) Methods in Computational Molecular Physics., D. Reidel Publising Company, Dordrecht, p. 189 (1983). 

The electron densities, bond orders, first six excitation energies, oscillator strengths, and weighting factors of pyrido[l,2-f)]pyridazinium cation were calculated by the PPP semiempirical version of the SCFMO-CI method, which indicated that protonation is expected to take place at the nonbridgehead nitrogen, and nucleophilic substitution is predicted to occur at position 3 (68TCA417i... 

---