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vibrational_perturbation_theory_vpt2 [2020/06/11 18:17] 127.0.0.1 external edit 
vibrational_perturbation_theory_vpt2 [2020/07/15 15:36] (current) qianli 
====== Vibrational perturbation theory (VPT2) ======  ====== Vibrational perturbation theory (VPT2) ====== 
 
''VPT2'',//options// [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. The current VPT2 implementation is limited to asymmetric top and linear molecules. For further details see:\\  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, //Semiquartic force fields retrieved from multimode expansions: Accuracy, scaling behavior and approximations//, [[https://dx.doi.org/10.1063/1.4918587J. Chem. Phys.]] **142**, 154118 (2015).\\  R. Ramakrishnan, G. Rauhut, //Semiquartic force fields retrieved from multimode expansions: Accuracy, scaling behavior and approximations//, [[https://dx.doi.org/10.1063/1.4918587J. Chem. Phys.]] **142**, 154118 (2015).\\ 
 
The following //options// are available:  The following //options// are available: 
 
 * **''COMBI''=//n//** (=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=1''. 
 * **''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 DarlingDennison resonances. 
 * **''DRTFC''=//value//** (=0.01 Default) Threshold for DarlingDennison 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''=//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''=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.  ''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.  * **''UBOUND''=//value//** (=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 ''COMBI'' calculations. 
 * **''USERMODE''=//n//** (=0 Default) Once this keyword has been activated (''USERMODE=1'') the list of states to be calculated is controlled by the ''VIBSTATE'' program. 
 
==== Example ====  ==== Example ==== 
mp2  mp2 
 
surf,start1D=label1,type=qff,sym=auto !(3) generate a QFF  {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  poly,type=qff,vam=0 !(4) transform the PES to polynomials 
vpt2 !(5) do a VPT2 calculation  vpt2 !(5) do a VPT2 calculation 
</code>  </code> 
 