In quantum chemistry, a complete active space is a type of classification of molecular orbitals.
The resulting wavefunction is of multireference nature, and is blessed by additional properties if compared to other selection schemes.
The active classification can theoretically be extended to all the molecular orbitals, to obtain a full CI treatment.
A Complete Active Space wavefunction is used to obtain a first approximation of the so-called static correlation, which represents the contribution needed to describe bond dissociation processes correctly.
Dynamic correlation, representing the contribution to the energy brought by the instantaneous interaction between electrons, is normally small and can be recovered with good accuracy by means of perturbative evaluations, such as CASPT2 and NEVPT.