Comparative study between two quantum spin systems KCuCl$_{3}$ and TlCuCl$_{3}$

We have performed an {\it ab initio} study of the electronic structure of two isostructural quantum spin systems, KCuCl$_{3}$ and TlCuCl$_{3}$, 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 TlCuCl$_{3}$ a {\it strongly} coupled s=1/2 dimer system compared to KCuCl$_{3}$ which shows a {\it weakly} coupled s=1/2 dimer behavior. Good agreement with the existing analysis of inelastic neutron scattering results has been observed.