Understanding the flat band in 1T−TaS2 using a rotated basis

The emergence of electronic flat bands in moir\'e superlattices has attracted much attention. Actually, a charge density wave (CDW) superlattice can play the same role, and 1$T$-TaS${}₂$ represents an example. Although there has been a lively debate on the strongly correlated physics in 1$T$-TaS${}₂$, a transparent theoretical understanding of the starting point -- the single-electron band dispersion -- is still elusive. This Letter shows how to understand the flat band in this compound with a rotated basis, a simple trigonometric function, and a 3$\ifmmode\times\else\texttimes\fi{}$3 matrix.