The non-empirical valence–electron (NEVE) method for molecular orbital calculations on molecules containing heavy atoms has been extended to include fourth-row atoms in a minimum STO basis calculation. Calculations on the halogens F2, Cl2, Br2 and I2, and the diatomic interhalogens CIF, BrF, BrCl, IF, ICl and IBr give results which are comparable with those of available ab initio calculations. Calculation of the Koopmans' defect for these molecules shows that the defect is only significant for the fluorine-containing molecules, and that the resulting orbital sequence agrees with the experimental photoelectron spectroscopic data in all cases. Calculated spin-orbit splittings also correlate with the observed values.