Complete 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:

• core, always hold two electrons
• active, supposed to hold zero, one or two electrons
• virtual, always hold zero electrons

This classification allows to develop a set of 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.

The active classification can theoretically be extended to all the Molecular orbitals, to obtain a Full CI treatment. In practice, this choice is limited, due to the high computational cost needed to perform a large CAS wavefunction on medium sized molecules.

Complete Active Space wavefunction is used to obtain a first approximation of the static correlation.