Please read section 9.6 and 9.6.2.
A little bit more complex are bulk calculations with internal degrees of freedom. The non ideal hcp phase (i.e. c/a not ideal) is a simple example for this case. The avoid problems occurring due to Pulay stress it is the safest to relax at constant volume. Add the line
NSW = 5 IBRION = 2 ISIF = 4 optional parameters not required POTIM = size of trial step for ions (try the default 1.0)to the INCAR file and use a UNIX batch file to calculate the equilibrium cell shape for different volumes. The batch file might look similar to
rm WAVECAR for i in 3.7 3.8 3.9 4.0 4.1 do cat >POSCAR <<! C: hcp $i 1.00000 0.00000000000000 0.00000 -0.50000 0.86602540378444 0.00000 0.00000 0.00000000000000 1.63299 2 direct 0.00000000000000 0.00000000000000 0.000000 0.33333333333333 0.66666666666667 0.500000 ! echo "a= $i" ; vamp E=`tail -1 OSZICAR` ; echo $i $E >>SUMMARY.hcp done cat SUMMARY.fccIn the param.inc file increase NGX,Y,Z to 16, and NGX,Y,ZF to 24. Also set NIOND=2 and NBANDS=16 and recompile VASP.
Exercise 6: If you want to relax the volume as well, always use a large cutoff. Usually 1.5* default cutoff is sufficient. Recreate the param.inc file with the makeparam utility program. Look up the ISIF parameter and set it correctly. Start an relaxation relaxing all degrees of freedom simultaneously.