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How to do geometry optimization with defect?

Total energy, DFT+U, BigDFT,...

raul_l
Posts: 74
Joined: Sun Jan 08, 2012 7:45 pm

How to do geometry optimization with defect?

I'm studying a material with 52 atoms in the unit cell. For the pure material I can do structural optimization and calculate other things just fine. However, when I substitute one of the atoms with an impurity atom, structural optimization is unable to converge. That is, the maximal force value (maxfor) goes like this:

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``2.539E-022.003E-021.416E-021.231E-021.480E-021.319E-021.930E-022.289E-022.301E-02...``

Is it possible that it has something to do with changes in symmetry? I can see the following in my log file:

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``--- !COMMENTmessage: |    The Bravais lattice determined only from the primitive    vectors, bravais(1)=  4, is more symmetric    than the real one, iholohedry=  1, obtained by taking into    account the atomic positions. Start deforming the primitive vector set.src_file: symbrav.F90src_line: 201... symlatt : the Bravais lattice is mC (one-face-centered monoclinic)--- !COMMENTmessage: |    The Bravais lattice determined from modified primitive    vectors, bravais(1)=  2, has a lower symmetry than before,    but is still more symmetric than the real one, iholohedry=  1    obtained by taking into account the atomic positions.src_file: symbrav.F90src_line: 210``

The pure material material has the tetragonal lattice. With a defect, I think it becomes monoclinic or triclinic. Should I modify the lattice constants and/or the primitive vectors slightly to help break the symmetry?
Raul Laasner
Netherlands Institute for Space Research

raul_l
Posts: 74
Joined: Sun Jan 08, 2012 7:45 pm

Re: How to do geometry optimization with defect?

It seems to have been just a really difficult structure to relax. With ionmov 7 it converged after about 130 cycles. Using that as input, it also converges with ionmov 2.
Raul Laasner
Netherlands Institute for Space Research