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Abstract: In this work, we investigate the influence of correlating core electrons on the relativistic corrections to the equilibrium geometries, harmonic frequencies and dissociation energies of hydrogen astatide, using moderately large basis sets. Our results suggest that relativistic effects tend to increase bond length and decrease harmonic frequencies and dissociation energies, in line with previous studies involving valence correlation. In all cases the driving force to the relativistic effects is the spin–orbit coupling, with scalar effects acting cooperatively except for the bond lengths. Core correlation counteracts the relativistic increase of bond lengths, but not strongly enough to reverse any trends. [Copyright &y& Elsevier]