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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