From VASP Wiki

NBLOCK = [integer]
Default: NBLOCK = 1 

Description: After NBLOCK ionic steps the pair-correlation function and the DOS are calculated and the ionic configuration is written to the XDATCAR-file. It is recommended to leave NBLOCK to 1, since the computational overhead to determine the DOS and pair correlation function is minimal. Only for molecular dynamics simulations with many 1000 steps or machine learning, it might be expedient to increase NBLOCK to say 10 or even 100, to avoid large XDATCAR-files and the evaluation of the pair correlation function at every step.

In addition

  • NBLOCK controls how often the kinetic energy is scaled if SMASS = -1.
  • After KBLOCK * NBLOCK ionic steps the averaged pair correlation function and DOS are written to the files PCDAT and DOSCAR. The internal accumulators are reset, and after another KBLOCK * NBLOCK steps the new averaged quantities are written out.
Mind: The CPU costs for these tasks are quite small, so use NBLOCK = 1, except when you apply machine-learned force fields. In that case, you might want to increase NBLOCK to 10 to 100 to decrease the computational load and the file size of the XDATCAR file.
Warning: The product of KBLOCK and NBLOCK should not be larger than the number of steps NSW. If KBLOCK * NBLOCK > NSW before starting the main ion loop then KBLOCK is automatically reset to 1. Next, if the same conditions is still true, NBLOCK is reset to NSW. Also, mind that NBLOCK will be at minimum ML_OUTBLOCK in MLFF prediction-only MD runs.

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