Doubly excited configuration

Let us consider another example. In describmg the n electron pair of an olefin, it is important to mix in doubly excited configurations of the fomi (n ). The physical importance of such configurations can again be made clear by using the identity... 

For /2(Ar)i7, an extensive CI-CSP simulation was carried out, and the results were compared with those of the simple CSP approximation. Both calculations are for the ultrafast dynamics following excitation of the I2 into the B state. We found that the CI-CSP calculation, including doubly excited configurations , is close to converged for times up to t 500 fs, when 1500 configurations are included. Fig. 2 shows co(t)p, the coefficient of the CSP term and the doubly excited terms in the full CI-CSP wavefunction,... 

The amount of computation for MP2 is determined by the partial transformation of the two-electron integrals, what can be done in a time proportionally to m (m is the number of basis functions), which is comparable to computations involved in one step of CID (doubly-excited configuration interaction) calculation. To save some computer time and space, the core orbitals are frequently omitted from MP calculations. For more details on perturbation theory please see A. Szabo and N. Ostlund, Modem Quantum Chemistry, Macmillan, New York, 1985. 

Due to the low lying 1 b LUMO (the p orbital centered on the carbon atom), the doubly excited configuration... 

C showing all configurations which arise from the C(3P) + H2 products. You can assume that doubly excited configurations lie much ( J 00 kcal/mole) above their parent configurations. 

Notice that the first two terms correspond to the Hartree-Fock energy, equation (A. 66). The last term is the sum of all doubly excited configurations. 

The method is based on the following procedure.33 All possible doubly excited configurations are generated from the Hartree-Fock function and their contributions to the second-order Rayleigh-Schrodinger perturbation theory energy computed. Approximately 100 of the most important are used for a Cl calculation, all singly... 

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