NBSEBLOCKV

NBSEBLOCKV = [integer] 

Default: NBSEBLOCKV

= -1

Description: NBSEBLOCKV specifies the blocking factor for the unoccupied states when setting up the BSE Hamiltonian.

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By default, the construction of the BSE Hamiltonian in VASP is parallelized over k-points, such that each MPI rank can compute a pair of k-points. This way the BSE Hamiltonian setup can be parallelized up to $\mathrm{NKPTS*(NKPTS+1)/2}$ MPI ranks or $\mathrm{NKPTS*2*(NKPTS*2+1)/2}$ for spin-polarized case, where $\mathrm{NKPTS}$ is the total number of k-points in the full Brillouin zone. However, if a large number of MPI ranks is used in a calculation with too few k-point, this leads to load imbalance, where some of the MPI rank will have no data to compute. In such cases, it is recommended to use parallelization over bands. If the paralliziation over bands is used, all occupied (unoccupied) bands are divided into a $\mathrm{NBLKO (NBLKV)}$ number of blocks, where $\mathrm{NBLKO=NBANDSO/NBSEBLOCKO}$ and $\mathrm{NBLKV=NBANDSV/NBSEBLOCKV}$. This bands blocking allows VASP to parallelize the setup of the matrix up to $\mathrm{NBLKO*NBLKV*NKPTS*(NBLKO*NBLKV*NKPTS+1)/2}$ ranks.

If no $\mathrm{NBSEBLOCKV}$ or $\mathrm{NBSEBLOCKO}$ is specified, no paralliziation over bands is used and $\mathrm{NBLKO=1 (NBLKV=1)}$.

Mind: Parallelization over bands with NBSEBLOCKV does not work with the old BSE driver, i.e., IBSE=0

Related tags and sections

BSE, NBSEBLOCKO, BSE calculations

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