I am trying to learn how to perform a (reasonable) ACFDT calculation in vasp 6 (not having any experience on RPA calculations in vasp 5 either). In order to check that we correctly understand all relevant keywords, we tried to reproduce old results first. As we are ultimately interested in metallic systems, we chose the lattice constant of Al as first test-case and tried to follow PRB 81, 115126 (2010) (this is the RPA lattice constant paper by Harl, Schimka and Kresse that basically also describes the implementation in VASP 5). Currently, we thereby extract the lattice constant by a fit to an EOS, rather than by trying to optimize the geometry, which (I believe) should be possible for metallic systems as of vasp 6.2.
However, we ran into several issues/questions that were not quite clear to us (me):
Table III of the paper states the parameters used in the final calculations. For Al, it states E_cut^EXX=360; k^EXX=20; E_cut^RPA=300; E_cut^\chi=200; k^RPA=12 (in bold letters). In the caption, it says: bold values for k mean that the same k-point sampling was applied for EXX and correlation energy. This triggered several questions:
- If the same k-point grid is used for EXX as for the correlation energy, why is then a separate k^EXX listed? Which parameter would this correspond to (nkred should decrease the number of k-points between the EXX calculation and the correlation calculation I believe, which is obviously not what one wants for metals)?
- Unless I am very mistaken E_cut^\chi is set in vasp by default to 2/3 of the E_cut^\RPA via the parameters ENCUTGW (corresponding to E_cut^\chi) and ENCUT (corresponding to E_cut^\RPA), which is consistent with E_cut^chi/E_cut^RPA=2/3. But how would a different E_cut^EXX be achieved (particularly when using the 1-step routine in vasp 6)? Is this achieved indirectly by setting NBANDSEXACT to a value lower than the max. number of plane waves? If so, considering the fact that NBANDSEXACT is not documented, is it still advised to play with E_cut^EXX vs. E_cut^RPA, or has it become general practice to use all bands available (i.e., the same Ecut for the EXX calculation as for the correlation)?