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 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 indicates what occupied orbitals to remove in the construction of the Hartree-Fock potential.
Read Specification of orbital strings of orbitals to be included in the Hartree-Fock potential when constructing the cationic Fock operator.