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:
“‘Tis chastity, my brother, chastity.
She that has that is clad in complete steel,
And like a quivered nymph with arrows keen
May trace huge forests and unharbored heaths,
Infamous hills and sandy perilous wilds,
Where, through the sacred rays of chastity,
No savage fierce, bandit, or mountaineer
Will dare to soil her virgin purity.”
—John Milton (1608–1674)
“In this lucid and flexible pattern only one thing remained always stationary, but this fallacy went unnoticed by Martha. The blind spot was the victim. The victim showed no signs of life before being deprived of it. If anything, the corpse which had to be moved and handled before burial seemed more active than its biological predecessor.”
—Vladimir Nabokov (1899–1977)
“A set of ideas, a point of view, a frame of reference is in space only an intersection, the state of affairs at some given moment in the consciousness of one man or many men, but in time it has evolving form, virtually organic extension. In time ideas can be thought of as sprouting, growing, maturing, bringing forth seed and dying like plants.”
—John Dos Passos (1896–1970)