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I run some calculations using xTB. From one of the outputs I got:

*** convergence criteria satisfied after 182 iterations ***

         #    Occupation            Energy/Eh            Energy/eV
      -------------------------------------------------------------
         1        2.0000           -0.6285969             -17.1050
       ...           ...                  ...                  ...
       254        1.7813           -0.3250849              -8.8460
       255        1.7322           -0.3248659              -8.8401
       256        1.4992           -0.3241340              -8.8201
       257        1.3002           -0.3236807              -8.8078
       258        1.1554           -0.3233899              -8.7999
       259        1.1298           -0.3233404              -8.7985
       260        0.9584           -0.3230132              -8.7896 (HOMO)
       261        0.9131           -0.3229267              -8.7873 (LUMO)
       262        0.8611           -0.3228267              -8.7846
       263        0.7655           -0.3226383              -8.7794
       264        0.6860           -0.3224747              -8.7750
       265        0.6470           -0.3223914              -8.7727
       ...           ...                  ...                  ...
       516                         17.7730979             483.6306
      -------------------------------------------------------------

Question: how can be the occupancy of molecular orbitals equal/above the LUMO greater than zero?

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  • $\begingroup$ Why does your LUMO have so high occupancy (almost close to 1)? I think probably your system has static correlation, or maybe the SCF did not converge to the correct solution (182 iterations is too high I feel). $\endgroup$
    – S R Maiti
    Sep 14 at 9:50
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    $\begingroup$ @SRMaiti, about the occupancy, this is my point. Turning off the Fermi smearing, return the integer occupancies: HOMO and below, 2, and LUMO and above, 0. About the number of iterations, my system is a boron-nitride (120 atoms) + 4 nikel atoms nanotube and the calculations were run with double the accuracy as default. $\endgroup$
    – Camps
    Sep 14 at 12:56
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XTB uses Fermi Smearing by default at 300K. If you disable it, you should find that the virtual orbitals once again have zero occupancy.

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    $\begingroup$ +1! I see you've been here for more than a year but haven't posted yet, so thank you for writing your first answer and we appreciate your contributions! We hope to see much more of you in the future!! I made a minor edit to your answer, just let me know if it's an error! $\endgroup$ Sep 12 at 15:39
  • $\begingroup$ Thanks Koben. I just put the temperature equal zero. In the output now appears the integer occupancy but the MOLDEN output file has the result with fractional values. $\endgroup$
    – Camps
    Sep 13 at 13:31

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