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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).

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.

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