Real-Space-BSE

Official Resources

• Source Code: https://github.com/AlexBuccheri/Bethe-Salpeter
• Developer: Alex Buccheri
• License: See Repository

Overview

Real-Space-BSE is a specialized Fortran 2003 library implementing the Bethe-Salpeter Equation (BSE) in a real-space formalism. Unlike many plane-wave or k-space codes, this tool is specifically designed for large molecular systems, utilizing a localized basis set to efficiently compute optical absorption spectra and optical band gaps for systems with thousands of atoms.

Theoretical Methods

• Real-Space Formulation: Solves the BSE directly in real-space coordinates.
• Minimal Tight-Binding Basis: Uses a simplified basis for efficiency.
• Two-Center Approximation: Approximates integrals to speed up computation.
• Full BSE: Solves the full equation, not restricted to the Tamm-Dancoff Approximation (TDA).
• Monopole Approximation: Used for Coulomb terms beyond nearest neighbors.

Capabilities

• Optical Spectra: Calculation of optical absorption cross-sections.
• Optical Band Gaps: Direct determination of optical gaps including excitonic binding.
• Large Systems: Tested on molecular systems up to 6000 atoms (as of 2017).
• Molecule-Only: Specifically for finite molecular systems, not periodic solids.

Implementation & Tech Stack

• Language: Fortran 2003 (F2003 library).
• Status: Research code / Library.
• Parallelization: Real-space methods are naturally parallelizable, though specific MPI/OpenMP details depend on the driver.

Application Areas

• Large Molecules: Organic semiconductors, polymers, large biomolecules.
• Nanostructures: Finite quantum dots or clusters where periodic boundaries are artificial.
• Excitonics: Study of spatial exciton extent in real space.