openmaterials.ai / codes / ShengBTE

ShengBTE: the phonon Boltzmann equation solved by iteration

A Fortran and MPI solver that takes second- and third-order force constants and iterates the linearized phonon BTE to convergence with adaptive broadening and no free parameters.

What it is and how it works

Full iterative solution

Starting from the relaxation time approximation, each iteration feeds the deviation of every mode back into the scattering sum until the conductivity converges. Si needs 5 steps and gains about 4% over RTA (Li 2014).

Adaptive Gaussian broadening

Energy deltas become Gaussians whose width is set per process from group velocities and grid spacing, scaled by scalebroad. The default 1.0 needs no tuning (Li 2014).

Three input files

CONTROL (namelists &allocations, &crystal, &parameters, &flags), FORCE_CONSTANTS_2ND from Phonopy or espresso.ifc2, and FORCE_CONSTANTS_3RD from thirdorder.py (Li 2014).

thirdorder.py

A Python script that builds the minimal set of displaced supercells for the anharmonic constants from point group and permutation symmetry, within a cutoff (Li 2014).

Isotopes, polar bonds, nanowires

Isotope scattering from natural abundances, non-analytical correction from Born charges and dielectric tensor, and nanowires with diffusive boundaries (Li 2014).

Per-mode outputs

BTE.omega, BTE.v, BTE.cv, BTE.w_anharmonic, BTE.w_isotopic, BTE.P3, BTE.kappa_tensor and BTE.cumulative_kappa_* against mean free path (Li 2014).

What people do with it

  • 172W/(m K), isotopically pure Si at 300 K, exp. 156Li 2014
  • 20%RTA underestimate of kappa in lonsdaleiteLi 2014
  • 5iterations to converge the BTE for SiLi 2014

Giant anharmonicity in SnSe

Inelastic neutron scattering plus ShengBTE on 3x5x5 supercells of 600 atoms show kappa converges only when third-order constants past 6 A include the anharmonic Sn-Se-Se triplets (Li 2015). Li 2015

MoSi2N4 monolayers

ShengBTE, on force constants from moment tensor potentials, gives iterative lattice conductivities of 440 W/m/K for MoSi2N4 and 500 W/m/K for WSi2N4 monolayers (Mortazavi 2021). Mortazavi 2021

How it is used

  1. Harmonic constants

    Phonopy gives FORCE_CONSTANTS_2ND (or Quantum ESPRESSO espresso.ifc2), with Born charges.

  2. Anharmonic constants

    thirdorder.py turns DFT forces on displaced supercells into FORCE_CONSTANTS_3RD.

  3. Write CONTROL

    Set natoms, ngrid, lattvec, positions, scell, T and flags such as isotopes or nanowires.

  4. Run ShengBTE

    Launch under MPI in the directory with the three files. No command-line arguments.

  5. Read the BTE.* files

    BTE.KappaTensorVsT_CONV, scattering rates, BTE.cumulative_kappa_tensor.