[Pw_forum] Relax calculation

Filipe Camargo Dalmatti Alves Lima flima at if.usp.br
Sat Apr 26 16:36:26 CEST 2014

The relax option does NOT optimize the lattice constants.  (useful when you
want to relax impurities of molecules)

The vc-relax option optimize the lattice constants. (useful only you have
crystalline system)

Now it depends what you want to do:

Do you have just a crystal? I should say vc-relax is enough.

Do you have a slab with a molecule attached into the surface? I would first
vc-relax the surface alone; I would insert the molecule, then I would
optimize the geometry using relax option.

If I had just a single molecule, I would create a very large box and just
relax the structure.

In order to be sure that your simulation is ok you must always check the
convergence parameters: total energy, k-point mesh, DOS, forces, etc... all
the time.
Also, keep in mind the convergence depends on the property you want to

Is there anything in particular that you have an issue?



On Fri, Apr 25, 2014 at 6:14 PM, Sohail Ahmad <sohailphysics at yahoo.co.in>wrote:

> I am using QE 5.0.2 and I have a very basic question.
> To optimise the structure of a material, i started with known experimental
> lattice constants and calculated energy vs lattice to know the optimised
> lattice constants.
> Now i wish to apply the strain
> so i increased lattice constant 'a' by 2% (let us say)
> now i need to relax (i believe) to get optimised coordinates before
> proceeding further and wish to know force and stress as well.
> So my question is i should consider it as 'relax' or "vc relax"
> if relax then following should work but unfortunately no
> tprnfor = .TRUE.
> tstress = .TRUE.
> ion_dynamics = 'bfgs'
> If i donot consider force and stress, then it gives me final coordinates
> Is the above step true in this particular set of problems of strain
> application or should i use variable cell.
> i am little confused.
> So
> ---------------------------------------------------------------------
> *Sohail Ahmad*
> King Khalid University
> Abha, Saudi Arabia
> --------------------------------------------------------------------
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Filipe Camargo Dalmatti Alves Lima
PhD Student
University of São Paulo, Physics Institute, Materials Physics Department,
Nanomol Group, Brazil.
Phones:    (11) 3091-6881  (USP)
                (11) 97408-2755 (Vivo)
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