`VCI` calculations account for vibration correlation effects and use potential
energy surfaces as generated from the `SURF` program and a basis of `VSCF` modals.
All `VCI` calculations will be performed state-specific, i.e. for each vibrational mode an individual
`VCI` calculation will be performed. As `VCI` calculations may require substantial computer resources, these
calculations can be rather expensive.
Currently, two different `VCI` programs (configuration selective and conventional) are available (see below).
Moreover, VCI calculations can be performed using the grid-based version of the program or within
a polynomial representation. The latter is significantly faster and is thus recommended.
The different versions of the
configuration selection `VCI` program and the underlying configuration selection scheme are described in
detail in:

M. Neff, G. Rauhut, *Toward large scale vibrational configuration interaction calculations*,
J. Chem. Phys. **131**, 124129 (2009).

M. Neff, T. Hrenar, D. Oschetzki, G. Rauhut, *Convergence of vibrational angular momentum terms within the Watson Hamiltonian*,
J. Chem. Phys. **134**, 064105 (2011).

The anharmonic frequencies and intensities calculated by the `VCI` program can be used to plot an
IR spectrum, using the `PUT` command (see subsection 10.3) with the *style* `IRSPEC`.

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molpro@molpro.net 2018-06-21