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##

Bulk calculations with internal parameters

Please read section 7.6 and 7.6.2.

Slightly more involved are bulk calculations with
internal degrees of freedom.
The non ideal hcp phase (i.e. c/a non ideal)
is a simple example for this case.
To avoid problems due to Pulay stress, it is
safest to relax at a set of constant volumes. Add the lines

ENCUT = 200.00 eV # energy cut-off for the calculation
PREC = Normal # Normal precision
LREAL = .FALSE # real space projection .FALSE. or Auto
EDIFFG = -0.01 # threshold for forces and stress tensor
EDIFF = 1E-5
NSW = 10 # 10 ionic steps
IBRION = 2 # CG algorithm
ISIF = 4 # forces and stress are optimized
# 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.fcc

*Exercise 5:*
If you want to relax the volume as well, always use a large cutoff.
Usually 1.3 times the default cutoff is sufficient.
Start a relaxation allowing all degrees of freedom to relax simultaneously.
Is the volume identical to the manual search. Repeat the calculations
at the default cutoff. How large is the error in the volume.

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at