Modified virtual orbitals.
The virtual canonical Hartree-Fock orbitals are not the optimal choice for correlated calculations in which the virtual space is truncated; the lower orbitals tend to be diffuse since they see the N + 1 electron system. One may exploit the freedom of rotation amongst the virtual orbitals to obtain orbitals better suited for correlated calculations. The default option in DIRAC when .MVO has been specified is to generate the modified virtual orbitals by construction of a Fock operator for the system with some electrons removed, as first proposed by Bauschlicher [Bauschlicher1980]. The orbital string specified with .VECMVO specifies which occupied orbitals to include in the construction of the modified Hartree-Fock potential for the virtual orbitals.
Note that the resulting modified virtual orbitals are no longer canonical and can not be used in MP2 calculations; they can be used in the various CI and CC modules, and as start orbitals for *KRMCSCF.
Read Specification of orbital strings of orbitals to be included in the Hartree-Fock potential when constructing the cationic Fock operator.