what is the difference between vertical ionization potential and adiabatic ionization potential and how can we calculate it by gaussian?
This topic was already discussed here:
https://www.researchgate.net/post/I_am_using_GAUSSIAN09_and_would_like_to_calculate_the_IPionisation_potential_and_EAelctron_affinity_on_a_neutral_molecule_Is_this_possible
A definition of vertical and adiabatic ionization energy can be found here:
http://w3.iams.sinica.edu.tw/lab/wbtzeng/labtech/ie_adia&vert.htm
Within Hartree-Fock theory, the first vertical ionization energy (VIE) of a closed-shell system can be calculated from the HOMO eigenvalue (Koopmans' theorem). Since the change in the number of electrons and the correlation energy are not correctly evaluated in a single-determinant description, remarkable errors can be found. Please look at this thread for the extension of Koopmans' theorem to DFT:
https://www.researchgate.net/post/Why_is_the_Koopmann_theorem_not_valid_for_DFT_calculations
VIEs can be calculated as the difference between the total electronic energies calculated for the starting compound (at its optimized geometry) and the corresponding cation (at the same geometry as the starting compound).
The adiabatic IE (AIE) can be calculated by taking into account the geometry relaxation of the cation. Hence, the ZPE-corrected total electronic energy of the fully relaxed cation should be considered in this case.
I hope this may help.
Dear Preeti,
that's correct. Pay attention to use the geometry optimized for the neutral species to calculate the geometry of the cation in VIE calcs.
In Gaussian (g16) this can be done as follows:
%oldchk=optimisedneutralcompound.chk
%chk=nonoptmisedcation.chk
#p b3lyp chkbasis geom=checkpoint guess=read pop=full
comment
1,2
This will keep the geometry of the neutral diamagnetic compound and apply it to the radical cation generated upon vertical ionization.
For the calculation of the adiabatic ionization energy, you will add the opt keyword.
Do not hesitate to contact me for any further information.
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