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How can I simulate the work function given by Ultraviolet Photoelectron Spectrum (UPS) of a molecule or a molecular crystal?

I thought it is an easy job: Just do a geometry optimization and get the energy of the neutral molecule and then using the same geometry to get the energy of the cation, the difference in energy can be the UPS given ionization energy. However I read a review, where what UPS can measure is discussed.

It is said that what UPS gives is like $$\phi_{\rm m}=E_{\rm vac}(s)-E_{\rm F},$$ instead of $$E_{\rm vac}(\infty)-E_{\rm VB},$$ which can be computed using the procedure I described. The symbols are

  • $E_{\rm vac}(s)$: vacuum level of a finite-size sample
  • $E_{\rm vac}(\infty)$: vacuum level at infinity
  • $E_{\rm F}$: Fermi level
  • $E_{\rm VB}$: top of valence band, or in molecular system, energy of neutral species, or, if we stuck to molecular orbital theory, HOMO.

I am confused in the following points

  1. Why the Fermi level instead of HOMO are measured since in some case no electrons occupies Fermi level?
  2. How can I compute the $E_{\rm vac}(s)$ using quantum chemistry software packages?
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  • You can use Galore package.

  • Galore is a package which applies Gaussian and Lorentzian broadening to data from ab initio calculations, such as VASP.

  • The two main intended applications are:

    • Gaussian and Lorentzian broadening of electronic density-of-states, with orbital weighting to simulate UPS/XPS/HAXPES measurements.

    • Application of Lorentzian instrumental broadening to simulated Raman spectra from DFPT calculations.

  • On the tutorial page, you will find a UPS example with SnO$_2$.

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    $\begingroup$ Thx. I think I get some points from your answer. The DOS is corresponding to the UPS. However, my main concerns on this topic are not clear. I edit my question so that people can notice my confusion. $\endgroup$ – Y. Zhai Nov 14 '20 at 7:01

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