The ABINIT Users Mailing List ( CLOSED )

["Aldo Humberto Romero Castro" <aromero@qro.cinvestav.mx>]

Hello Anurag

> I
> wanted to ask few question about cell relaxation.  For some compounds I
> wish to calculate I do
> not have the exact space group and atomic positions so at times I take
> an iso-structural cell and relax the cell with "optcell 2" and specify
> a dilatmx value (also ionmov 2).  But I see that abinit relaxes the
> lattice vectors (a,b,c) and also the angles alpha,beta, gamma.  could
> you explain how beneficial it is use changed alpha/beta/gamma values in
> addition to the lattice vectors ?  I was using the changed lattice
> vectors but original angles.

You should always start with optcell 0, it is safer (probably not faster).
Remember that you are starting with a cell which is close to the one
you want but of course there are differences between the predicted one
and yours, they could depend on many things besides physics, such as
pseudopotential type, exchange correlation etc. Therefore, I usually
start with opcell 0, then optcell 1 and look at the inner stress, if they
are still large, you will need to go to optcell 2. Since version 5.4
(I believe), the cell is symmetrized and it tries to be as close as possible
to the symmetry you have started from. At the end, you will have to be
close to zero stress in your system

>
> I am not quite sure if I understand the guidelines on the abinit webpage
> regarding "It is STRONGLY suggested first to optimize the ionic
> positions
> without cell shape and size optimization (optcell=0),
> then start the cell shape and size optimization
> from the cell with relaxed ionic positions."  Should I be doing a ionic
> position optimization (optcell 0) before optcell 2 ?
>

Naturlich!

> Since
> ultimately I am interested to dope the material (replace one cation with a
> isovalent dopant ion)
> later on so I was wondering how much beneficial it is to follow the
> above mentioned recipe.  When I relax the structure with a dopant I
> only do ionic position relaxation (ionmov 2) keeping the cell volume
> constant (optcell 0).
>

Just remember that after your optimization and depending on your
dopant, you will have to do the whole process again, the cell can
expand or contract, accordingly. Of course, take care of possible
interaction between dopants due to the periodic boundary conditions,
in case you do not want that..


Good Abinit calculation..

regards

-- 
-------------------------------------------
Prof. Aldo Humberto Romero
CINVESTAV-QUERETARO
Libramiento Norponiente No 2000
Real de Juriquilla
76230 Queretaro, Qro, Mexico

email:  aromero@qro.cinvestav.mx
Phone:  (52)-442-211 9909
Fax: (52) 442 211 9938
www:  www.qro.cinvestav.mx/~aromero