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# Vibrational perturbation theory (VPT2)

`VPT2`

,*options* [vpt2]

The `VPT2`

program is based on force constants, which are retrieved from the polynomial coefficients as generated by the `POLY`

program. Therefore, each `VPT2`

calculation requests a call of the `POLY`

program prior to the `VPT2`

call. As the `VPT2`

program relies on a quartic force field (QFF), one may use the option `TYPE=QFF`

in the `SURF`

program. This will lead to tremendous time savings as the size of the potential energy surface is significantly reduced. However, this is an option and the force constants can be retrieved from any potential provided by the `SURF`

program. The current VPT2 implementation is limited to asymmetric top and linear molecules. For further details see:

R. Ramakrishnan, G. Rauhut, *Semi-quartic force fields retrieved from multi-mode expansions: Accuracy, scaling behavior and approximations*, J. Chem. Phys. **142**, 154118 (2015).

## Options

The following *options* are available:

`PRINT`

=*n*`PRINT`

=0 (default) prints the anharmonic vibrational frequencies and the most important vibrational constants.

`PRINT`

=1 prints in addition the force constants as retrieved from the polynomial coefficients.

`PRINT`

=2 prints the force constants and an analysis of the detected resonances.

`INFO`

=*n*`INFO=1`

provides a list of the values of all relevant program parameters.

### Example

The following example shows an input for water, for which a semi-quartic force field will be generated at the MP2/cc-pVDZ level.

memory,20,m basis=vdz orient,mass geometry={ 3 Water O 0.0675762564 0.0000000000 -1.3259214590 H -0.4362118830 -0.7612267436 -1.7014971211 H -0.4362118830 0.7612267436 -1.7014971211 } mass,iso hf mp2 optg !(1) optimizes the geometry frequencies,symm=auto !(2) compute harmonic frequencies label1 {hf start,atden} mp2 surf,start1D=label1,type=qff,sym=auto !(3) generate a QFF poly,type=qff,vam=0 !(4) transform the PES to polynomials vpt2 !(5) do a VPT2 calculation