:orphan:
 

star(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 :ref:`WAVE_FUNCTION_.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 :cite:`Bauschlicher1980`.
The orbital string
specified with :ref:`MVOCAL_.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 :ref:`*KRMCSCF`.

Keywords
========

keyword(PRINT)

Print level.

*Default:*

::

    .PRINT
     0

keyword(VECMVO)

Read :ref:`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.


keyword(INTFLG)

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