Bond lengths, harmonic vibrational frequencies and dissociation energies of TlAt and (113)(117) are calculated at the highly correlated level of theory with and without spin-orbit interactions. Spin-orbit effects of Group 13 and Group 17 elements almost cancel out for R-e The spin-orbit effects on D-e of TlAt(-1.20 eV) and (113)(117)(-2.05 eV) are roughly the sum of those of the corresponding element hydrides. Element 113 becomes more electronegative than Element 117 upon inclusion of spin-orbit interactions in the (113)(117) molecule. Electron correlations and spin-orbit effects are almost additive in the TlAt molecule, but the nonadditivity is quite pronounced for the (113)(117) molecule. (C) 2003 Elsevier Science B.V. All rights reserved.