How to perform GW calculations in VASP?

[sandeep.kumar]

I am trying to calculate band structure of a system containing two atoms per unit cell (graphene) using GW calculations. It is taking a lot of time and after 5 days it shows wall time exceeded.
I have done the self consistent procedure in 3 steps.
In 1st step i have used the INCAR file given as

# Basic setup:
SYSTEM = Graphene
ISMEAR = 0
SIGMA=0.05
EDIFF = 0.00000001

and used the POTCAR file of PBE potentials give as some few lines...

PAW_PBE C_h 06Feb2004
4.00000000000000
parameters from PSCTR are:
VRHFIN =C: s2p2
LEXCH = PE
EATOM = 147.1560 eV, 10.8157 Ry

TITEL = PAW_PBE C_h 06Feb2004
LULTRA = F use ultrasoft PP ?
IUNSCR = 1 unscreen: 0-lin 1-nonlin 2-no
RPACOR = 0.800 partial core radius
POMASS = 12.011; ZVAL = 4.000 mass and valenz
RCORE = 1.100 outmost cutoff radius
RWIGS = 1.630; RWIGS = 0.863 wigner-seitz radius (au A)


Then i copy the CONTCAR file of first step and repeat the calculations using INCAR file as

# Basic setup:
SYSTEM = Graphene unoccupied orbitals
ALGO=Exact
NELM=1
NBANDS=96
ISMEAR = 0
SIGMA=0.05
LOPTICS=.TRUE.

and same POTCAR file as given above.

Then i copied the CONTCAR, WAVECAR and WAVEDER file of second step and repeated the calculations using INCAR as

# Basic setup:
SYSTEM = Graphene GW
ALGO=GW0
LSPECTRAL=.TRUE.
NOMEGA=50
LRPA=.FALSE.
NBANDS=96


and same POTCAR file as above....

and 2nd and 3rd steps are taking a lot of time, 2nd step completed in 4 days on 1 processor and 3rd step even could not get completed after 4 days.
I have also done the calculations on 20 processors, even then it could not get completed, error was wall time exceeded.

Can someone please tell me, where i am doing the mistake. Its taking a huge time even on a very small system

[askhetan]

how big is your system ? How many ions ? How many electrons ? How many kpoints ? Can you share your POSCAR ?
GW calculations usually take a huge amount of memory and time and a lot of empty bands to get any meaningful values. Its virtually impossible to say anything about your system unless you mention your exact specifications.

[sandeep.kumar]

Here is my POSCAR file.

vasp
2.467
0.500 0.866 0.000
-0.500 0.866 0.000
0.000 0.000 10.000
2
Direct
0.333 0.333 0.000
0.666 0.666 0.000

and i have used Monkhorst pack for sampling K-space with a mesh of (25*25*1) in KPOINTS file.