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CASSCF (Read 6441 times)
Gerrit-Jan Linker
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24.12.07 at 13:23:57
CASSCF = Complete Active Space Self Consistent Field theory
In CASSCF chemically important "active' orbitals are selected. These lead to the MCSCF configurations obtained by a full CI within that active space.
In CASSCF a complete CI is performed over the active space.
In quantum chemistry, a Complete Active Space is a type of classification of molecular orbitals. Spatial orbitals are classified as belonging to three classes:
With CASSCF static correlation is accounted for to first approximation. Static correlation is important to take into account when describing bond dissociations for example.
Dynamic correlation is not accounted for in CASSCF. This effect is usually taken care of as a perturbation of the CASSCF wavefunction as CASPT2.

  • inactive orbitals, sometimes also called core orbitals or holes
    Doubly occupied in all configurations.
    Occupied orbitals excluded from the CAS expansion.
  • virtual orbitals, sometimes also called secondary orbitals or particles
    Unoccupied in all configurations.
    Unoccupied orbitals excluded from the CAS expansion.
  • active orbitals
    Partially occupied orbitals. All possible configurations are considered in the CAS expansion.
    This classification allows to develop a set of Slater determinants for the description of the Wavefunction as a linear combination of these determinants. Based on the freedom left for the occupation in the active orbitals, a certain number of electrons are allowed to populate all the active orbitals in appropriate combinations, developing a finite-size space of determinants.

    Performing a complete CI in the active space has the advantage that the orbital rotations among active orbitals are redundant and do not need to be considered.

    Quote from thesis Coen de Graaf  
    More information:
    Complete active space
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« Last Edit: 09.11.10 at 23:12:46 by Gerrit-Jan Linker »  

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