The ABINIT Users Mailing List ( CLOSED )

[Steven Erwin <erwin@dave.nrl.navy.mil>]

Dear Yudon Liu,

All you need to do, for the migration path from (000) to (111), is to 
use the simple constraint (again for the example of atom #16):

 nconeq 1 natcon 1 iatcon 16 wtatcon 1 1 1,

which will constrain atom #16 to move only within the plane 
perpendicular to the (111) direction.

Does that answer your question?

Regards,

Steve

ÁõÔ¥¶« wrote:
> Dear Steve:
> Very,very glad to get your letter! 
> Your help is desperately needed by me! I am in honor to be the second to use the
> constraint equation. Hahah
>
> I calculate the migration of interstitial atom from (000) to (1/2 1/2 1/2) along 
> [111] direction. So I wish to relax the interstitial atom at different (111) cross
> plane with the different sum___X(i)+Y(i)+Z(i)=0.1,0.2,...0.5. something like a
> series of  C.T. photoes. Then I link the stable sites at the different C.T.
> photoes to get the migration path. My crystal cell is rhombohedral, so its
> symmetry should let the atoms near interistial atom to also move inside a 
> cross
> plane like (111). 
>
> Your explaination tells me how to move the atom from (000) site to (100) site
> along x axils, that is very usful, and I will try it.  However in my case, my
> diffusion path is from (000) to body center ,then to (100). Thus, I perfer to 
> know
> how to contraint the atom in a cross plane like (111). 
> Now, can you tell me how to perform such kind of constraint?
>
> yours
>
> Yudong Liu
>
>
>
>
> > From: Steven Erwin <erwin@dave.nrl.navy.mil>
> > Reply-To: forum@abinit.org
> > To: forum@abinit.org
> > Subject: Re: [abinit-forum] how to use constrait equation in relaxing the
>
> atom?
>
> > Dear Yudong Liu,
> >
> > Since I contributed the geometry constraint subroutine (and may be the 
> > only user to date!), I will try to answer your question.
> >
> > The constraints are implemented in a fairly general -- although possibly 
> > confusing -- manner.  The input variable 'wtatcon' (WeighTs for AToms in 
> > CONstraint equations) tells the whole story:
> >
> > --
> > "Gives the weights determining how the motion of atoms is constrained 
> > during structural optimization or molecular dynamics (see nconeq, 
> > natcon, and iatcon). For each of the nconeq independent constraint 
> > equations, wtatcon is a 3*natcon array giving weights, W[I], for the x, 
> > y, and z components of each of the atoms (labeled by I) in the list of 
> > indices iatcon. Prior to taking an atomic step, the calculated forces, 
> > F[I], are replaced by projected forces, F'[I], which satisfy the set of 
> > constraint equations
> >
> > Sum[mu=x,y,z; I=1,natcon]: W[mu,I] * F'[mu,I] = 0 for each of the nconeq 
> > arrays W[I]."
> > --
> >
> > Your example -- relaxing an atom in a fixed plane -- is especially easy. 
> >  Use the following tokens/values to allow all atoms to relax while 
> > keeping  the z-coordinate of atom #16 frozen at its initial value:
> >
> > nconeq 1 natcon 1 iatcon 16 wtatcon 0 0 1.
> >
> > A slightly more complicated example would be the following.  Suppose you 
> >  have a surface in which two nominally equivalent atoms (say, #15 and 
> > #16) MIGHT have a broken-symmetry ground state in which their heights 
> > are different.  You could, of course, just choose your surface unit cell 
> > large enough to allow this, perturb the heights of these two atoms, and 
> > then relax everything.  But a more systematic approach would be to 
> > calculate the total energy as a function of their height difference. 
> > You can do this by the following setting:
> >
> > nconeq 1 natcon 2 iatcon 15 16 wtatcon 0 0 +1  0 0 -1,
> >
> > which enforces the constraint z(atom15)-z(atom16)=constant during a 
> > geometry relaxation.
> >
> > I hope this gives you enough of an answer to make some progress.  Please 
> > let me know if you need more information.  From the generality of your 
> > question, I guess the documentation wasn't very helpful to you.  This 
> > makes me think I should re-write the 'wtatcon' entry!
> >
> > Regards,
> >
> > Steve Erwin
> >
> >
> > liuyudong97@mails.tsinghua.edu.cn wrote:
> >
> > > Dear all:
> > > Abinit seems to permit the relax a atom in a fixed plane by employing the
>
> structural optimization command------nconeq, natcon, iatcon, and wtatcon.I 
> guess
> that equations' formulasim possible is xi+yi=0.5 or sth.
>
> > > Now,My question is: 
> > >
> > > How to use these command (or constrait equation) exactly? Can an expert show
>
> me an example?
>
> > > your help will be greatly appreciated!
> > >
> > > by
> > > yours
> > > Yudong Liu
> >
> > --
> > Steven C. Erwin, Code 6390                    erwin@dave.nrl.navy.mil
> > Center for Computational Materials Science           tel 202/404-8630
> > Naval Research Laboratory, Washington DC 20375       fax 202/404-7546
> > +++++++++++++++  http://cst-www.nrl.navy.mil/~erwin  ++++++++++++++++

--
Steven C. Erwin, Code 6390                    erwin@dave.nrl.navy.mil
Center for Computational Materials Science           tel 202/404-8630
Naval Research Laboratory, Washington DC 20375       fax 202/404-7546
+++++++++++++++  http://cst-www.nrl.navy.mil/~erwin  ++++++++++++++++