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I am performing SCF calculations using Quantum Espresso for crystalline materials to determine the band gaps and energies. A sample output for one of the crystals is shown below.

    highest occupied, lowest unoccupied level (ev):     9.2589   13.2824

!    total energy              =    -392.86522200 Ry
     estimated scf accuracy    <       0.00000034 Ry

     The total energy is the sum of the following terms:
     one-electron contribution =     -87.58728570 Ry
     hartree contribution      =      68.54396895 Ry
     xc contribution           =     -87.05600859 Ry
     ewald contribution        =    -248.69804485 Ry
     one-center paw contrib.   =     -38.06785182 Ry
      -> PAW hartree energy AE =       0.00000000 Ry
      -> PAW hartree energy PS =       0.00000000 Ry
      -> PAW xc energy AE      =       0.00000000 Ry
      -> PAW xc energy PS      =       0.00000000 Ry
      -> total E_H with PAW    =      68.54396895 Ry
      -> total E_XC with PAW   =     -87.05600859 Ry

I would like to know if the total energy here is an absolute or a relative value. Also, does lower total energy always imply more stability, or do I have to divide the energy by the number of atoms to compare the stabilities of different crystals?

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  • $\begingroup$ The absolute value of the total energy doesn't have any meaning. It can vary depending on the pseudopotential, software, convergence threshold, etc. See this and this. $\endgroup$
    – Abdul Muhaymin
    Aug 30 at 6:21
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    $\begingroup$ Thanks for the links. I understand that the absolute value depends on those factors. In my case, the pseudopotential, software, and convergence threshold are consistent for all the crystals I perform calculations. The answers in the links you sent suggest that it is okay to compare energy values in that case. So it still means that a crystal with a lower energy value is more stable than the one with a higher energy right? $\endgroup$
    – Prashant Govindarajan
    Aug 30 at 6:39
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    $\begingroup$ yes, I think so. Keeping everything else the same, if you change one parameter and look for the lowest energy value, that would be the most stable structure. If you change two parameters, then you have to find the minimum of the energy surface to get a stable structure (local minima=metastable, global minima=stable) $\endgroup$
    – Abdul Muhaymin
    Aug 30 at 6:44

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