CASSCF correlation energy

Along the whole 6 in 6 CASSCF IRC segments calculated for these two reactions, SC calculations (carried out at geometries corresponding to points on the respective IRCs) recover not less than ca. 93% of the 6 in 6 CASSCF correlation energy. This justifies our use of this combination of approaches to study the evolution of the electronic structure of each of the two reacting systems. 

Figure 11-9. CASSCF potential-energy profiles of the ground-state So (circles), the lnjr state (triangles), the Lb state (squares), and the La state (filled squares) of the 9H-adenine along the linear interpolation reaction path from the equilibrium geometry of the nit state to the CI32 (a) and CI16 (b) conical intersections. The diabatic correlation of the states is shown in (a). (From Ref. [138])...

Having decided about the basis set, we now turn to the problem of choosing an appropriate form of the wave function. Since we are going to calculate an energy difference between two electronic states, it is clear that we have to include as much correlation effects as possible. We shall do that in two steps In the first step we try to account for the near degeneracy effects by using the CASSCF method with an appropriate choice of the active space. On top of that we shall have to perform multi-reference Cl calculations to account for the dynamical correlation energy. 

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