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a) the electric-field gradient is obtained in a.u. (i.e., hartree/bohr2)

b) conversion factor "hartree/bohr2 --> J/m2" is 4.3597482*10-18/(5.29177249*10-11)2

c) the electric-field gradient is multiplied by the corresponding nuclear quadrupole moment. Those are typically given in mbarn (= 10-31 m2) and can be taken from the compilations of P. Pyykkö (Mol. Phys. 106, 1965 (2008)).

c) conversion to Hz, i.e., divide by h: 1/6.6260755*10-34

In total, this means that the electric-field gradient (in a.u.) is multiplied by the quadrupole moment (in mbarn) and a factor of 234.96474 to obtain the quadrupole coupling in kHz.