to everybody:
3 days in a row I was testing everything that my imagination allowed to and came
up to a conclusion that with soft materials, with orthorhombic cells of > 20
atoms there are TOO many degrees of freedom to expect vc-relax to give good
numbers for cell parameters.
Within the same third decimal digit in total energy (-xxxx.xxX Ry) one might
came to slightly(?!) different cells depending on starting points and general
logic of relaxation (say, you start from orthorhombic space group doing relax
and for a corresponding ibrav continue with vc-relax Or you start from the
closest tetragonal space group and continue with xyz vc-relax).
I will also do a few variable-cell NEB (coupled with QE for ab initio part)
between the orthorhombic and tetragonal systems, maybe it will help me more to
better understand the problem.
PS I have a full right to be wrong I am learning and will be thankful for any
feedback ;-)
Dear Alex,
we made this experiment: we created 10 initial configuration by randomizing
the lattice spacings by 5% and we performed very tight vc-relax-ations.
Then we took the average lattice spacings and angles with their standard
deviations. For a typical orthorhombic perovskite with 20 atoms, we got
+/-0.007 angstrom accuracy on the lattice, +/-0.05° on the angles and
+/-0.0003 Ry on the energy.
For a given XC functional, this is the accuracy due to the numerics.
We are doing worse than PXRD experiments! If your system is softer, I
would expect even larger standard deviations.
HTH.
Best,
Davide
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