Abstract
We have performed an ab initio study of the electronic structure of two isostructural quantum spin systems, KCuCl3 and TlCuCl3, which have recently attracted much attention due to their unconventional magnetic properties. Our first-principles analysis shows unambiguously the role of Tl, as opposed to structural differences between the two compounds, in making TlCuCl3 a strongly coupled s = ½ dimer system compared to KCuCl3 which shows a weakly coupled s = ½ dimer behavior. Good agreement with the existing analysis of inelastic neutron scattering results has been observed.