almaBTE

Official Resources

• Repository: https://bitbucket.org/sousaw/almabte
• Documentation: http://www.almabte.eu/
• License: Apache License 2.0

Overview

almaBTE is a high-performance C++ software package designed for calculating the lattice thermal conductivity and other phonon transport properties of materials from first principles. It solves the Peierls-Boltzmann Transport Equation (BTE) for phonons, moving beyond the relaxation time approximation (RTA) to capture full scattering processes, including three-phonon interactions. It is particularly adept at handling multiscale systems, from bulk crystals to thin films, superlattices, and alloys.

Scientific domain: Phonon Transport, Thermal Conductivity, Nanoscale Heat Transfer Target user community: Researchers in thermoelectrics, thermal management, and phononics

Theoretical Methods

• Phonon BTE: Solves the linearized BTE for the phonon deviation distribution function.
• Iterative Solver: Self-consistent solution to capture Normal (N) and Umklapp (U) scattering processes.
• Second & Third-Order Force Constants: Inputs derived from DFT (via codes like VASP, Quantum ESPRESSO, or Phono3py).
• Kappa Decomposition: Resolves thermal conductivity by mode, mean free path, and frequency.

Capabilities

• Bulk Properties:
  • Lattice thermal conductivity tensor ($\kappa_{\alpha\beta}$).
  • Cumulative $\kappa$ vs. MFP.
  • Phonon lifetimes and line widths.
• Nanostructures:
  • Thin films (Cross-plane and In-plane).
  • Superlattices (effective medium or explicit).
  • Nanowires (diffuse boundary scattering).
• Disorder:
  • Virtual Crystal Approximation (VCA) for mass disorder (isotopes/alloys).

Key Strengths

• Beyond RTA: One of the few publicly available codes that rigorously solves the full scattering matrix inversions.
• Multiscale: Can treat ballistic-diffusive regimes in nanostructures.
• Interface: User-friendly Python/XML inputs and HDF5 outputs.
• Efficiency: Parallelized with MPI and OpenMP for large q-grids.

Inputs & Outputs

• Inputs:
  • FORCE_CONSTANTS_2ND and FORCE_CONSTANTS_3RD (from Phono3py/ShengBTE format).
  • XML input file defining the grid and temperature.
• Outputs:
  • HDF5 files containing mode-resolved properties.
  • Text summaries of thermal conductivity.

Interfaces & Ecosystem

• Upstream:
  • Phono3py: Common generator for the necessary force constants.
  • VASP/QE: Source of force calculations.
• Downstream:
  • Plotting scripts included for visualization.

Advanced Features

Multiscale Capabilities:

• Thin film thermal conductivity (cross-plane and in-plane)
• Superlattice modeling (effective medium and explicit)
• Nanowire transport with boundary scattering
• Ballistic-diffusive transport regimes

Disorder Treatment:

• Virtual Crystal Approximation (VCA)
• Isotope scattering effects
• Alloy mass disorder
• Phonon-defect scattering

Advanced BTE Solutions:

• Full scattering matrix inversion
• Beyond relaxation time approximation
• Normal and Umklapp processes
• Mode-resolved analysis

Performance Characteristics

• Computational Cost: Dominant cost is the input DFPT/Force calculations. The BTE solution itself is relatively fast (minutes to hours).
• Scaling: Good MPI scaling for the BTE solver.
• Parallelization: MPI and OpenMP support
• Memory: Efficient HDF5 storage

Computational Cost

• Force constants: DFT calculations dominate (external)
• BTE solution: Minutes to hours depending on grid
• Nanostructure calculations: Additional cost for geometry
• Overall: Efficient once force constants available

Comparison with Other Codes

• vs. ShengBTE: almaBTE is considered a successor or alternative, with better object-oriented design (C++) and more features for nanostructures (thin films).
• vs. Phono3py: Phono3py focuses on the force constants and simple BTE; almaBTE offers more flexible BTE solvers (e.g., for superlattices) on top of those constants.

Best Practices

Input Preparation:

• Use high-quality force constants from phono3py
• Validate bulk properties before nanostructure calculations
• Check convergence with q-point density
• Test different boundary conditions

Calculations:

• Start with RTA for quick estimates
• Use full BTE for accurate results
• Monitor convergence of iterative solver
• Validate against experimental data

Application Areas

• Thermoelectrics: Designing materials with low $\kappa_L$ to high $zT$.
• Thermal Management: Understanding heat flow in semiconductor thin films.
• Isotope Engineering: Effect of isotopic purity on diamond/Si thermal conductivity.
• Nanostructured materials: Thin films, superlattices, nanowires
• Phononic engineering: Thermal conductivity reduction

Community and Support

• Development: Developed by Jesus Carrete, Bjorn Vermeersch, and collaborators.
• Source: Bitbucket repository
• License: Apache License 2.0
• Documentation: http://www.almabte.eu/
• Support: Issue tracking on Bitbucket
• Publications: Well-cited in thermal transport community
• User base: Thermoelectrics and thermal management researchers

Verification & Sources

• Repository: https://bitbucket.org/sousaw/almabte
• Primary Publication: Carrete et al., Comp. Phys. Comm. 220, 351 (2017).
• Verification status: ✅ VERIFIED
  • Active and widely cited.
  • Considered a standard tool in the field.