Relaxation time approximation model predictions for the thermal properties of Si, Si0.99Ge0.01 and Si:B at room temperature (Supplementary Equation 2). (a) Thermal conductivity accumulation functions, α. First-principles results from ref. 27 and ref. 28 for Si are included for comparison (green dashed and solid lines). (b) Spectral distribution of the heat capacity. (c) Spectral distribution of the thermal conductivity.