On Fri, 23 Nov 2012 17:41:19 +0100, Amin Torabi <mtorabi at uwo.ca> wrote:
> Dear Layla > > You said that "ecutvcut is a parameter which is not strictly bonded to > hybrid functional parameters, but to strongly anisotropic supercells" > So, why do we have ecutvcut=0.7 in the PBE0 example for bulk Si? > > Hmm I think there seems to be a lot of confusion here. The first thing to understand is that the value ecutvcut has no effect if exxdiv_treatment is set to anything other than 'vcut_ws'. By default all of the EXX examples run with exxdiv_treatment set to 'gygi-baldereschi'. So in all these examples the value of ecutvcut is redundant. However with extra command line options to run_example it is possible to run the examples with different exxdiv_treatment values. Hence why ecutvcut is set to some default value (0.7) In the case of HSE however using vcut_ws (or vcut_spherical) is a rather dubious thing to do at best (due to the fact the functional already separates the exchange interaction into long and short range and only uses true non-local exchange in the short range part.) As for the meaning of ecutvcut: vcut_ws truncates the Coulomb potential *used in the exact-exchange integrals* in real space according to the extent of the Wigner-Seitz cell (taking into accoutn k-point sampling). The way it does this is by building the interaction on a real space grid and the fourier transforming it, right at the start of the calculation. This interaction (no in G space) is used throughout the rest of the calculation for the EXX terms. ecutvcut is the cutoff used in this transform and has to strike a balance between producing an accurate enough 1/|r-r'| function in G space and being so large as to take up too much time at the start of the calculation. Brgds Simon -- Simon Binnie | Post Doc, Condensed Matter Sector Scuola Internazionale di Studi Avanzati (SISSA) Via Bonomea 256 | 34100 Trieste | sbinnie at sissa.it
