The documentation for PySCF can be found here: https://sunqm.github.io/pyscf/

However, it does not explicitly state in the documentation what unit of energy is used. Is there a way to find this out?


Same as any other electronic structure code: atomic units are used. The unit of energy is Hartree, $E_h$, in which the ground state energy of hydrogen is exactly $-0.5$. One Hartree is roughly 27.211 386 245 988 eV, see the NIST database. The atomic unit for length is the Bohr radius, $a_0$.

Defining things in terms of atomic units has the huge advantage that the results of a calculation don't depend on the numerical values of various conversion factors, e.g. Å to bohr in the input geometry conversion, and hartree to eV in the energy printout...

  • 5
    $\begingroup$ It should be noted that some codes (VASP) use electron volts. $\endgroup$
    – Wychh
    Jul 27 '20 at 9:04
  • 2
    $\begingroup$ Well yes, but in this case the output usually says eV. Atomic units are dimensionless, which makes them an optimal choice for computer implementations. $\endgroup$ Jul 27 '20 at 9:06
  • $\begingroup$ A good point :) $\endgroup$
    – Wychh
    Jul 27 '20 at 9:07

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