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Hi friends ,i am trying to construct an Energy volume plot for delta zncl2, in order to calculate the equilibrium lattice constant.I am using the tags:

IBRION=2
ISIF=2
ISMEAR=0
PREC=High
NSW=100

I gave relaxations and re-relaxations until the CONTCAR and POSCAR files exactly matched..I then used the final TOTEN value from the OUTCAR file[without entropy] and plotted it against volume for different values of lattice constant (a,b,c i have varied c keeping the other 2 constant).
However,i am not getting an energy minimum in the region around the experimental lattice constant even though external pressure ->0 around experimental lattice constant.Energy minimum comes only around volume corresponding to ~(lattice constant+0.5) angstroms.
Have I missed a step? is something left to be done?am i reading the wrong energy value?please suggest.
The manual says i need to give a run with ISMEAR=-5 and no relaxations.Can somebody please provide the other parameters of the required INCAR file(especially ISIF)?

1) automatic cell shape and volume relaxation can be done by ISIF=3
(not 2)
2) mind that the equilibrium lattice parameters depend on the chosen potential (LDA,GGA,...) type.
3) please read the accuracy.pdf tutorial (to be downloaded from the VASP-workshop site) on how to perform geometry optimizations.