Nuclear size effects in rotational spectra: A tale with a twist

Molecular field shift: 4-Component relativistic calibration calculations show a coincidence of errors in previous theoretical and experimental studies of nuclear volume effects in the rotational spectra of diatomics. The central quantity for the calculation of corrections to the rotational Dunham coefficient Y01 is the derivative of the electron density with respect to internuclear distance, here shown for the TlI molecule.