V m ⇤ b , where N V is the valley degeneracy. The quality factors are then calculated using Eq. 15, where µ 0 and  L at 300 K are calculated using parameterized semi-empirical models (see Refs. 29, 40 for details). The bulk modulus, which is an input parameter for the semi-empirical models, is calculated by fitting the the Birch-Murnaghan equation of state to a set of total energies at di↵erent volumes. In each case, we adjust the Fermi energy to maximize zT at 300 K.