Useful Keywords

This is a short summary of useful keywords in the context of large-scale calculations. Not all of them were mentioned during this tutorial, but may be worth a try for your large-scale workflow.

use_local_index: A must-set keyword for all simulations with more than 100 atoms. In any large-scale run, this keyword should be used. This keyword ensures that the memory usage per task (!) does not needlessly scale O(N^2) - a scenario that would otherwise prevent the code from running any larger systems. use_local_index can lead to slight slowdown (test to see if this is even the case) but can actually also facilitate speed-ups for large cases.
load_balancing: Can lead to a small speedup when used together with use_local_index. Should usually work but using use_local_index on its own is also fine.

Things that were not discussed above but may be worth checking:

preconditioner kerker off: For periodic runs, a so called preconditioner is used in the mixing. For systems that are safely non-metallic, this may not be needed and, in large runs, can save noticeable amounts of time. However, note this does need testing especially for very large runs.
elsi_restart keyword: If you are trying to restart a single scf cycle for a very large system, you can use the electronic restart. However, be careful, e.g. you must use ALL k-points when using GGA restart files to start a hybrid run. (In scalar-relativistic DFT-GGA runs of periodic systems, FHI-aims, by default, uses time reversal symmetry to reduce the number of k-points. In order to use all k-points (no symmetry use in the Brillouin zone at all) in preparation of writing restart files for a subsequent hybrid DFT run, set symmetry_reduced_k_grid .false.)
frozen_core_scf keyword: In simulations dominated by the eigensolver ("solution of K.-S. equations"), you can achieve substantial speedup at essentially no accuracy loss ¹ by "freezing" core orbitals in the eigenvalue solver. E.g., to freeze all core orbitals deeper than -200 eV, use:
```
frozen_core_scf .true.
frozen_core_scf_cutoff -200
```
dry_run keyword:

What good is a "dry run" (i.e., a test of the input files before running the actual calculations)?

Upon starting the FHI-aims calculation, the code will first check the input keywords in both control.in and geometry.in for consistency and (hopefully) provide an error message in case of any typographic errors or known inconsistent keyword combinations that would prevent a correct run of FHI-aims.

In many real world scenarios, FHI-aims will be run through a queueing system, i.e., everything will be set up, a job will be queued, the user goes away and waits for the supercomputer to execute the code. In this case, stopping with a trivial error can cost serious amounts of real-world time since the error will need to be corrected and the task re-queued.

FHI-aims allows one to check the consistency of input files at the command line without executing the full run if the keyword

dry_run

is included in the control.in file. In that case, the code will just go through all motions to set up the calculation but not actually execute the time consuming parts.

If you're so inclined, modify your control.in file to include the dry_run keyword and see what happens if you execute FHI-aims at the command line. This should tell you if the input files would be accepted for a later, real-world run, helping you to identify incorrect keywords or typos ahead of time.

Yu, Victor Wen-zhe, Jonathan Moussa, and Volker Blum. "Accurate frozen core approximation for all-electron density-functional theory." The Journal of Chemical Physics 154.22 (2021): 224107. ↩