[QE-users] Optimal QE setup for molecular crystals (pseudopotentials, k-space campling, performance) ..

[Husak Michal]

Hi all ...


Before I continue I would like to mentioned I had done the homework on this topic (checked mail list, studied about 20 related articles, done a few hundred test calculations by QE and CASTEP) ...

The target of my work is to verify (enhance quality) of X-ray based molecular crystal structure solution

by DFT methods ...


Target molecules:  primary C,H,N,O composed middle size molecular crystals (100-500 atoms per unit cell).

Insulators. Non magnetic. Other atoms potentially: F,Cl,S,B,Na,


Goal: Confirm the geometry from X-ray is correct (by results RMS comparison with original) ..


Questions:

1) What pseudopotential is suitable ?  pbe-n-rrkjus_psl.1.0.0.UPF Like one ?


2) Some sources suggest 38 Ry cutf off (study done in VASP), H-ultrasoft potential suggest 46 Ry cutoff,

QE related study (M. Lund) suggest 55 Ry cut off ...

I need to screen a lot of structures = be fast = low cut off . Opinion ?


3) How can i determine I had chosen correct parameters ?

Comparison of RMS with X-ray data is not reliable - X-ray shows position of max. electron density,

DFT shows positions of cores (especially for H-atoms)  ...

Any idea how to benchmark, before doing some large screening ?

Does exist some database of "100% reliable determined" reference structures ?


4) Gamma K-point only usage will be nice.

M. Lund work suggest 2x2x2 k-point to be OK  (is it identical to K_POINTS automatic  2 2 2 0 0 0 ) ?

What do you thing ?


Thanks for any ideas


Michal Husak


UCT Prague