DOLO (Do Localized Orbital) CASSCF
CASSCF can capture static electron correlation (i.e. orbital degeneracies), and it is very important to select the active space which includes the degenerate orbitals. For example, in a bond breaking event the $\sigma$ and $\sigma$* orbital will become degenerate and should be in the active space. However, anyone familiar with CASSCF will know how difficult it is to get the active space that you want. Particularly, if you are trying to solve for orbitals which are not the frontier orbitals. CASSCF can have many solutions, and sometimes you have to rotate really weird orbitals into the active space region and even that is not guaranteed to work and you might ask,... why? The reason is orbital rotations which are involved in the SCF part mix the orbitals. So even if you start with something that you like, you aren't guaranteed to keep that.
So what can we do?
In DOLO convergence is imposed with the condition that the orbitals remain “as similar as possible” to the initial orbital, i.e. it avoid total diagonalization of Fock or Density matrices. Therefore, nice localized orbitals can be used and they remain nice localized orbitals.
For some nice free pictures see these slides. Also the original reference:
Direct generation of local orbitals for multireference treatment and subsequent uses for the calculation of the correlation energy, Daniel Maynau, Stefano Evangelisti, Nathalie Guihéry, Carmen J. Calzado and Jean-Paul Malrieu, J. Chem. Phys. 116, 10060 (2002). My favorite example, is the dissociation of a C-H bond in ethylene. Without DOLO these requires the full valence active space. But with DOLO the minimal (2,2) active space can be used.
Note, I haven't actually used DOLO. It can be found here, in the COST package from Toulouse. But why isn't this more popular?!