====== Vibrational perturbation theory (VPT2) ======

''VPT2'',//options//

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 directive ''%%VTAYLOR,TYPE=QFF%%'' in the ''XSURF'' 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 ''XSURF'' program. As the results may be sensitive to thresholds for the resonance treatment, the user may be careful concerning these. For further details see:\\

R. Ramakrishnan, G. Rauhut, //Semi-quartic force fields retrieved from multi-mode expansions: Accuracy, scaling behavior and approximations//, [[https://dx.doi.org/10.1063/1.4918587|J. Chem. Phys.]] **142**, 154118 (2015).\\

The ''VPT2'' is currently limited to asymmetric top and linear molecules. By default the ''VPT2'' program will compute the fundamental transitions, but overtones and combination bands can be specified in the ''VIBSTATE'' program (being described in the ''VSCF'' manual). 

===== Options =====

The following //options// are available:

  * **''DIPOLE''=//n//** ''DIPOLE=1'' provides dipole moment surfaces to the VPT2 program and thus allows for the calculation of infrared intensities.
  * **''DRTFREQ''=//value//** (=300.0 Default) Frequency threshold for Darling-Dennison resonances.
  * **''DRTFC''=//value//** (=0.01 Default) Threshold for Darling-Dennison resonances concerning quartic force contants.
  * **''FRTFREQ''=//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).
  * **''FRTFC''=//value//** (=0.01 Default) Threshold for Fermi resonances concerning cubic force constants (type 1 and type2).
  * **''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.


==== 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.

<code>
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
</code>