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, partially occupied orbitals
- virtual, always hold zero electrons
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. 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. In practice, this choice is limited, due to the high computational cost needed to optimize a large CAS wavefunction on medium and large molecular systems.
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. This requires a wavefunction that includes a set of electronic configurations with high and very similar importance. 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.
Famous quotes containing the words complete, active and/or space:
“Although those notes, in conformity with custom, come after the poem, the reader is advised to consult them first and then study the poem with their help, rereading them of course as he goes through its text, and perhaps after having done with the poem consulting them a third time so as to complete the picture.”
—Vladimir Nabokov (1899–1977)
“You need not be proud of me.... I’m only being active till you can be again—it isn’t such a great desire on my part to serve the world and I’ll fall back into habits of sloth quite easily!”
—Eleanor Roosevelt (1884–1962)
“True spoiling is nothing to do with what a child owns or with amount of attention he gets. he can have the major part of your income, living space and attention and not be spoiled, or he can have very little and be spoiled. It is not what he gets that is at issue. It is how and why he gets it. Spoiling is to do with the family balance of power.”
—Penelope Leach (20th century)