ABINIT Discussion Forums

hello,
I have a unit cell with only one type of atoms (pure element) with a defined space group. Now, suppose that I want to add a solute atom (in a substitutional way) and optimize this new structure. If I let abinit cheking the symmetries (nsym=0) it will find a less symmetric space group than the one of the pure element, and if I optimize like this, I will go away from my first structure. At the end I will lose my geometrical symmetries (without regarding the type of atoms) at such a point that even if I replace the solute atom in the relaxed structure with the one of the pure element abinit will never recognize the initial space group.
In such situation may I say that I still have a substitutional solute atom ? no, because the whole lattice has been changed.
so my question is:
how may I optimize the structure with a solute atom and keep the lattice symmetries as if the structure is still pure ?

sorry to be so long.
thanks.

Could it be that you should use a supercell : your cell with a substitute atom should be surrounded with 7 or 35 or (n^3-1)... cells without a substitute atom. The symmetry will still vanish but the concentration of substitute atom will diminish and thus its effect on the cristal structure.

thank you for replying.
yes that seems a good idea for a dilute solute defect calculation.
but what if the solution is not dilute and I still want to relax while keeping my initial symmetries of pure element.

I can not really understand your concerns :
Either the structure you input is really unstable and there is no reasons for Abinit to preserve it under structural optimisation ; or you used some wrong calculation parameters and you must correct them. A last possibility would be that the cristal structure you try to simulate is unfit for pseudopotentials-planewaves-dft and you should use an appropriate quantum chemistry methods.

Moreover I should underline the following : Abinit never breaks symmetries unless ordered to. If I understand correctly your calculation, you have broken some initial cristal symmetry with a substitute atom. So Abinit keeps the remaining symmetries and performs the relaxation. What else could be expected ? Is there something I am misunderstanding ?
So If I'm understanding correctly, the answer to your initial question :

how may I optimize the structure with a solute atom and keep the lattice symmetries as if the structure is still pure ?

Is :
No way. The initial symmetry is broken by the solute atom. There is nothing you or anyone could do about that. Sorry. Or, you should imagine a structure with your solute atom that has the correct symmetry. But is that really what you want to do ?

indeed, I should have not been very clear.
here is a more explicit (I hope) version:
suppose that I have a special or general equivalent wyckoff position with a multiplicity m. An atom siting in such position should be repeated m times and the different positions are related by the symmetries of the space group. when optimizing the structure the symmetries are respected via some kind of mathematical constraints applied to all atoms of the considred wyckoff positions, i.e if an atom should be moved due to some acting force all other atoms related to it should follow in such a way to conserve the symmetries and movements breaking symmetries are forbidden. it's an artificial way (even if physically motivated) to prevent structures from collapsing under optimization.
Now suppose that I replace one, two or more atoms (less than m) in the considered wyckoff position with another type of atoms: symmetries are broken and the former mathematical constraints are no longer applied.
Now I just want to recover all the lost mathematical constraints applied before I replace the atoms, even if the symmetry is actually broken, it's no more than a mathematical constraint (physically motivated for me).

thank you for your time and attention.