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 _______________________________________________ Quantum ESPRESSO is supported by MaX (www.max-centre.eu) users mailing list users@lists.quantum-espresso.org https://lists.quantum-espresso.org/mailman/listinfo/users
