Converge EPS_SCHWARZ and CUTOFF_RADIUS for hybrid functionals

[Max H]

Dear all,

I plan on doing some single point calculations with the PBE0 functional and a ccGRB basis and would appreciate advice on the selection of appropriate parameters, because I have not found information of sufficient detail.

I read in the recent review (JPhysChemB, "The CP2K Program Package Made Simple") and in various other sources that the EPS_SCHWARZ and CUTOFF_RADIUS parameters are crucial for accuracy and computational efficiency.

What is the typical approach to converge these values? Are single-point calculations with one SCF cycle sufficient as soon as energy convergence is observed (comparable to the convergence protocols for CUTOFF and REL_CUTOFF)?

When studying metallic systems, which values of CUTOFF_RADIUS can be expected?
Supposing that the CUTOFF_RADIUS did not converge at half the smallest cell dimension, is the best approach then to set MULTIPLE_UNIT_CELL 2 2 1 (xy periodic system) in the &CELL block or do special restrictions arise?

I appreciate your kind advice.

Best,
Max

[Frederick Stein]

Dear Max,

Both parameters are determined differently.

CUTOFF_RADIUS needs to be less than half of the distance of an atom to its nearest image. For sufficiently large cells, you may even reduce it to values at the order of 5-7 A (please check convergence of your results in that case). If this parameter is not yet converged, you employ the MULTIPLE_UNIT_CELL as you suggested. Convergence is expected if CUTOFF_RADIUS can be set to values of at least 5 A.

EPS_SCHWARZ is determined by checking convergence of total energies. A good starting point is 1E-6 but larger values may be necessary, especially if you set "SCREEN_ON_INITIAL_P T" or with ADMM (ADMM is recommended).

Best,

Frederick