:orphan:
 

.. index:: *MVOCAL
.. _*MVOCAL:

========
\*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
========

.. index:: .PRINT
.. _MVOCAL_.PRINT:

.PRINT
------

Print level.

*Default:*

::

    .PRINT
     0

.. index:: .VECMVO
.. _MVOCAL_.VECMVO:

.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.


.. index:: .INTFLG
.. _MVOCAL_.INTFLG:

.INTFLG
-------

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