Complete active space state interaction

Xe and Rn elements Table 1.3. The scalar spin-free lOTC complete active space CASSCF/CASPT2 method was used followed by the restricted active space state-interaction (RASSI) method with the use of the atomic mean-filed spin-orbit (SO) integrals AMFI. 

The relative energies of the four states can of course be computed by performing a complete active space configuration interaction (CASCI) calculation with the active orbitals being (p and relative energies by considering the physics of the system. 

Some molecular species are not calculated properly by straightforward model chemistries these include singlet diradicals and some excited state species. For these the standard method is the complete active space approach, CAS (CASSCF, complete active space SCF). This is a limited version of configuration interaction, in which electrons are promoted from and to a carefully chosen set of molecular orbitals. 

Nondynamical electron correlation effects are generally important for reaction path calculations, when chemical bonds are broken and new bonds are formed. The multiconfiguration self-consistent field (MCSCF) method provides the appropriate description of these effects [25], In the last decade, the complete active space self-consistent field (CASSCF) method [26] has become the most widely employed MCSCF method. In the CASSCF method, a full configuration interaction (Cl) calculation is performed within a limited orbital space, the so-called active space. Thus all near degeneracy (nondynamical electron correlation) effects and orbital relaxation effects within the active space are treated at the variational level. A full-valence active space CASSCF calculation is expected to yield a qualitatively reliable description of excited-state PE surfaces. For larger systems, however, a full-valence active space CASSCF calculation quickly becomes intractable. 

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