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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

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.

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.