We present an ab initio theoretical approach to accurately describe phonon thermal transport in semiconductors and insulators free of adjustable parameters. This technique combines a Boltzmann formalism with density functional calculations of harmonic and anharmonic interatomic force constants. Without any fitting parameters, we obtain excellent agreement ͑Ͻ5% difference at room temperature͒ between the calculated and measured intrinsic lattice thermal conductivities of silicon and germanium. As such, this method may provide predictive theoretical guidance to experimental thermal transport studies of bulk and nanomaterials as well as facilitating the design of new materials.
The 2nϩ1 theorem and the density-functional perturbation theory have been used to calculate anharmonic force constants completely ab initio. Explicit expressions for the anharmonic coupling constants are presented, i.e., for the third-order derivatives of the total energy with respect to atomic displacements. Using the harmonic as well as the anharmonic results the phonon linewidth of Ge and Si as a function of temperature has been calculated for various branches and various ͑in particular, for nonvanishing͒ wave vectors completely ab initio.
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