I want to plot ionization spectra for Singly, doubly, and triply ionized molecule i.e. to be more specific is the x-ray spectra for singly to triply ionized atom or molecule using Gaussian or Orca ?
Gaussian is perfectly capable of calculating UV spectra. You can use a variety of methods for this job: ZIndo, TD-DFT (with a broad choice of density functionals), EOM-CCSD,CAS-CI. This is pretty straightforward. For starter I would suggest you TD-DFT; an exemplary input line:
#p TD=(singlets,NStates=10) CAM-B3LYP/6-31++G(d,p)
CAM-B3LYP functional with split valence basis set including polarization and diffuse functions are reasonable choices for a good start. Most of the time you should not be concerned of triplet states as well. If Gaussview does not display UV spectrum for some reason I would suggest checking the output file manually, and looking for "Excitation energies and oscillator strengths:" line. Beneath this line you should find the calculated data on electronic transitions. You can then proceed to draw the UV spectrum by yourself; here is a nice tutorial for doing this:
http://www.gaussian.com/g_whitepap/tn_uvvisplot.htm
However assigning peaks in UV spectrum is more complex procedure; for this you will need to work with checkpoint file data. Unfortunately at this moment I can not point you to any good reference on this matter. The best I could find is:
http://www.molcalx.com.cn/wp-content/uploads/2015/01/Gaussian09W_tutorial.pdf
see Chapter 9.3
Thanks Mohamed Khedawy , I know how to calculate UV spectra. But I am looking for X-ray spectra not UV spectra. In UV specta valance electrons get ejected while in x-ray core is ejected this is what all we know. so I want ionization spectra of core orbital up to 3rd ionization
XPS is in principle pretty straightforward, all you would need to do is take your orbital energies and put Lorentzian functions with the width that the setup you want to compare with has, that would be a simple Koopmans' theorem approach which was sufficient for most systems I came across so far. This, however, will require some sort of offset calculation because functionals are often optimized for bond lengths, relative energies and ionization potentials, not the deeper orbitals. For a more sophisticated approach that involves additional effects, see e.g.
Article Practical estimation of XPS binding energies using widely av...
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