I would like to calculate the vertical ionization energy of a molecule using Gaussian16. And which is the best method to do it?
Dear Suriyaprasanth,
To reach this purpose, suppose your molecular system (namely M) is first optimized in the gas phase at an appropriate level such as B3LYP/6-31G(d) to obtain corresponding total electronic energy. Then, you have to remove one electron from this optimized system to build a cationic species (M+). An additional single-point energy calculation over this cationic species gives you associated total electronic energy.
The Vertical Ionization Energy (VIE)= [The total electronic energy of cationic species (M+)] - [the total electronic energy of the initially optimized structure (M)]
This quantity would be obtained in Hartree. If you multiple this quantity by 627.51, you will obtain VIE in kcal/mol.
Note, however, not any geometry optimization should be performed when the initially optimized structure is converted into a cation (the cationic species does not need any geometry optimization).
About the method:
It should be noted that DFT-functionals are so weak to properly determin the energy of virtual levels (LUMO and LUMO+n). The HF method provides a considerably better estimation of virtual energy levels and you can use, for instance, HF/6-311++G(2df,2pd) level. Finally, if the molecular system of interest is not so large and you also have access to a powerful computational resource, the CCSD(T)/aug-cc-pVTZ or, even, CCSD(T)/aug-cc-pVQZ could be one of the best methods for your goal.
I hope this could be of helpful.
Regards,
Saeed
Dear Saeedreza Emamia,
Thank you very much for your clear explaination
By means of quantum chemical program package.
Please, concentrate on this paper (s.i. file):
Article Stochastic dynamic ultraviolet photofragmentation and high c...
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