# *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¶

### .PRINT¶

Print level.

*Default:*

```
.PRINT
0
```

### .VECMVO¶

Read Specification of orbital strings of orbitals to be included in the Hartree-Fock potential when constructing the cationic Fock operator. Default is the doubly-occupied molecular orbitals from the SCF.

### .INTFLG¶

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