How to Choose Dynamical Matrix Cutoff

GULP
1

I have two related questions, one related to GULP and one more lattice dynamics in general:

  1. In GULP is there a way to define the cut-off used to calculate the dynamical matrix / phonon frequencies as different from the cutoff in the interatomic potential?
  2. For three body potentials does it make sense for the force constant cutoff to be larger than the potential cutoff (for any order force constant)?

Thanks!

2
  1. You can do this by restarting. Run your optimisation with one cutoff and then restart as a single point calculation of the phonons with a different one. Of course, this is not a sensible thing to do since your phonons require that the structure be at a minimum, so you’re introducing an error into the calculation.
  2. The range of the force constants can’t be larger than the longest range force field term. Of course if you have electrostatics then this is long-range and will dominate.
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