modified Becke-Johnson potential (mBJ) for band gaps

by **Algerien1970** Fri 29 May - 18:34

From the userguide :

The modified Becke-Johnson exchange potential + LDA-correlation (Tran and Blaha 2009) allows the calculation of band gaps with an accuracy similar to very expensive GW calculations. It is a local approximation to an atomic ``exact-exchange''-potential and a screening term. This is just a XC-potential, not a XC-energy functional, thus modified Becke-Johnson potential (mBJ) for band gaps Img16

is taken from LSDA and the forces cannot be used with this option.

We recommend the following steps to perform such calculations:

run a regular initialization and scf cycle using LDA or PBE,
create case.inm_vresp (cp $WIENROOT/SRC_templates/case.inm_vresp case.inm_vresp.
edit case.in0 and set "R2V" option (instead of "NR2V").
run one more scf-cycle (use run_lapw -NI -i 1) to generate the required case.vresp* files.
``save'' the LDA (PBE) calculation.
edit case.in0 and change the functional to option indxc=28.
cp case.in0 case.in0_grr and change indxc in case.in0_grr to 50. This option will calculate the average of over the unit cell. (The presence of case.in0_grr will be detected during the scf-cycle and lapw0 will be called twice, first with the input file case.in0_grr, then with case.in0.)
run another scf cycle.

In most cases MSEC1 mixing will lead to convergence problems of the scf cycle. One should switch in case.inm to PRATT mixing, first using a smaller mixing factor (eg. 0.2), later increasing it to 0.50.