The ABINIT Users Mailing List ( CLOSED )

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

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  ++++++++++++++++