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Hi
I want to optimize the lattice parameters ( a and c/a) of a tetragonal structure manually. VASP can do it using some options, but I want to do it manually i.e. total energy minimization. I will be very thankful, if you someone let me guide that how can I proceed and how to optimize a and c/a of a tetragonal structure.
Thank you

if you want to keep the volume constant, this can be done quite easily:
write a small script that varies c/a over a certain range, plugs that into the Bravais matrix of POSCAR (give the atomic psitions in direct corrdinates!) and does a series of VASP static runs for each geometry.
Please remember that you can give the volume of the cell instead of the lattice constant in POSCAR by giving -V [Ã…3] in the second line
you can then automatize the runs like was done in loop.sh of VASP workshop example 2_1
If the volume is not to be kept constant, there has to be an additional (outer) loop running over different volumes as well.
In any case, please don't forget to use PREC=Accurate and high ENCUTs!