Interplane and Intraplane Coupling in Charge-Density-Wave Phases of $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $

At low temperatures, the layered transition metal dichalcogenides $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $ adopt similar charge-density wave structures corresponding to a $\sqrt {13} \times \sqrt {13} $ reconstruction in which the Ta atoms cluster within the triangular layers. Yet $\mbox{TaS}_2 $ also undergoes a metal-insulator transition, while $\mbox{TaSe}_2 $does not. Here we present a density-functional-theory study of the electronic structure of $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $. The half filled Ta d band at the Fermi level is found to differ significantly in the two materials: in $\mbox{TaSe}_2 $ the band is three dimensional, while in $\mbox{TaS}_2 $ it is highly one dimensional. These results are analyzed using maximally localized Wannier functions. The effects of stacking sequence and the spin-orbit interaction will also be discussed.