In most cases, the ``best'' results (i.e. closest to experiment) are obtained by iterating only G, but keeping W fixed to the initial DFT W. This can be achieved in two ways. If the spectral method is not selected (LSPECTRAL=.FALSE. requiring much more compute time), the QP shifts are iterated automatically four times, and you will find four sets of QP shifts in the OUTCAR file. The first one corresponds to the GW case, the final one to the GW results. The INCAR file is simply:
System = Si NBANDS = 96 ISMEAR = 0 ; SIGMA = 0.05 ALGO = GW0 ; NOMEGA = 50 ; LSPECTRAL=.FALSE.For technical reasons, it is not possible to iterate G in this manner if LSPECTRAL=.TRUE. is set in the INCAR file (this is the default). In this case, an iteration number must be supplied in the INCAR file using the NELM tag. Usually three to four iterations are sufficient to obtain accurate QP shifts.
System = Si NBANDS = 96 ISMEAR = 0 ; SIGMA = 0.05 ALGO = GW0 ; NOMEGA = 50 NELM = 4If non diagonal components of the self-energy (in the orbital basis) should be included use ALGO=scGW0, or equivalently ALGO=QPGW0 (as of VASP.5.2.13). The following setting can be used:
System = Si NBANDS = 96 ISMEAR = 0 ; SIGMA = 0.05 ALGO = scGW0 ; NOMEGA = 50 | or ALGO = QPGW0 NELM = 4In this case, the orbitals are updated as well by constructing a Hermitian (energy independent) approximation to the self-energy . The ``static'' COHSEX approximation can be selected by setting NOMEGA = 1 . To improve convergence to the groundstate, the charge density (and the charge density only) is mixed using a Kerker type mixing in VASP.5.2.13 and more recent versions (see Sec. 6.49). The mixing parameters AMIX, BMIX, AMIX_MAG, BMIX_MAG, AMIN can be adjusted, if convergence problems are encountered.
We strongly urge the user to monitor convergence by inspecting the lines ``charge density residual'' in the OUTCAR files. Alternatively the mixing may be switched off by setting IMIX=0 and controlling the step width for the orbitals using the parameter TIME (which defaults to 0.4). This selects a fairly sophisticated damped MD algorithm, that is also used for DFT methods when ALGO is set the ``Damped''. In general, this method is more reliable for metals and materials with strong charge sloshing.