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56 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 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. Note, the option QFF=1 in the POLY program has no impact on VPT2 calculations, but on VSCF and VCI calculations. The current VPT2 implementation is limited to asymmetric top 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 (2015) 154118.



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