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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. Once the `XSURF`

program has been used, the directive `VTAYLOR,TYPE=QFF`

should be called. 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. As the results may be sensitive to thresholds for the resonance treatment, the user may be careful concerning these. 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:

(=0 Default) Be default only fundamental frequencies will be computed. As many applications request also overtones and combination bands, the can be computed with`COMBI`

=*n*`COMBI=1`

.`DIPOLE`

=*n*`DIPOLE=1`

provides dipole moment surfaces to the VPT2 program and thus allows for the calculation of infrared intensities.(=300.0 Default) Frequency threshold for Darling-Dennison resonances.`DRTFREQ`

=*value*(=0.01 Default) Threshold for Darling-Dennison resonances concerning quartic force contants.`DRTFC`

=*value*(=500.0 Default) This is the threshold for Fermi resonance detection with respect to frequencies, i.e. $2\omega_i - \omega_k$ (type 1) and $\omega_i + \omega_j - \omega_k$ (type 2).`FRTFREQ`

=*value*(=0.01 Default) Threshold for Fermi resonances concerning cubic force constants (type 1 and type2).`FRTFC`

=*value*`INFO`

=*n*`INFO=1`

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

(=10000.0 Default) Upper limit of transition energies. This option can be used to limit the number of states to be computed in case of`UBOUND`

=*value*`COMBI`

calculations.(=0 Default) Once this keyword has been activated (`USERMODE`

=*n*`USERMODE=1`

) the list of states to be calculated is controlled by the`VIBSTATE`

program.

### 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 {xsurf,start1D=label1,sym=auto !(3) generate a QFF vtaylor,type=qff disk,where=home,dump='water.pot'} poly,type=qff,vam=0 !(4) transform the PES to polynomials vpt2 !(5) do a VPT2 calculation