# *MVOCAL¶

## Introduction¶

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*.

## Keywords¶

### .VECMVO¶

Read *Specification of orbital strings* of
orbitals to be included in the Hartree-Fock potential when constructing
the cationic Fock operator.

### .INTFLG¶

Specify what two-electron integrals to include during the construction
of the modified virtual orbitals (default: *.INTFLG* under ***HAMILTONIAN*).