On Tue, 21 Nov 2006, SuiYang wrote:
| Dear SIESTA users:
| I have used SIESTA to study the property of FePt systems.Here I have a
question about the atomic forces SIESTA calculated:
| When I am calculating the FePt unit cell(including 4 atoms) with its lattice
constant around 4 ang.The corresponding atomic forces are around 0.02
eV/ang.When I am using the 2x2x2 supercell(including 32 atoms) as unit cell,
with relatively the same atomic positions,and the lattice constant is changed
to 8 ang to maintain it is the same bulk as the previous one,however ,the
atomic forces are increased to around one hundred eV/ang.
|
| I also calculated another system using 3x3x3 supercell as unit cell, the
forces are much even higher ,it seems the atomic force will increase with the
number of atoms and the lattice constant.
|
| I am rather confused with its behavior,why there is such a drastic change in
atomic forces and stress tensor while the bulk is the same, only the expression
of it is different.
|
| I've attached both 1x1x1 and 2x2x2's input and output files,if needed.
| I really need your help.
|
Dear SuiYang,
As Riccardo pointed out, you are doing calculation with the default
setting for k-points, that is just one k-point. This is no good
when calculating bulk - in any case, metals. You can see
that Siesta fails to converge your system - which is in fact metal
with partially filled bands, - when using just one k-point and hence
no dispersion; it just desperately swaps the Fermi energy back and forth:
siesta: iscf Eharris(eV) E_KS(eV) FreeEng(eV) dDmax Ef(eV)
...
siesta: 23-3027.3747-2953.1394-2953.2110 0.8298 2.5907
...
siesta: 98-3034.0919-2952.4118-2952.4146 0.8258 -2.0656
siesta: 99-3027.3736-2953.3750-2953.4466 0.8258 2.5911
Normally you should be wondered by this behavior before going any further...
After you manage to get reasonable convergence of electronic
properties, take into account that the forces are especially sensitive
to the k-mesh, and (if you really need good forces, i.e. for phonons)
you should check how they converge as you increase the number of k-points.
Your forces are zero in your 4-atom cell,
because the positions of atoms are symmetric with respect to the
real-space mesh (defined by the Mesh cutoff, 200 Ry in your case),
and the errors cancel. In order to see how good/bad the forces REALLY
are, you can displace the atoms a bit from the symmetric positions,
imposing the uniform shift (AtomicCoordinatesOrigin).
In your 2x2x2-cell you have apparently
the atoms situated at random with respect to the mesh along
the Y-direction, and you see huge fluctuations of forces,
which - in principle, i.e. apart from numerical errors - should be zero
by symmetry:
siesta: Atomic forces (eV/Ang):
10.01 94.7209640.06
2 -0.04 -94.7209750.03
30.04 -117.510866 -0.02
4 -0.05 117.5108590.00
I must confess I don't understand why X and Y behave differently,
avan as they are completely equivalent with respect of atomic positions
and mesh construction. I'd be glad to see somebody's comment on that.
You did't need to provide .fdf separately because
they are dumped at the beginning of each Siesta output anyway.
I don't know what you are after in your study, but if I remember
correctly, in real life FePt is magnetic, so that when you switch on the spin,
this will affect your forces as well.
Hope this helps,
Andrei Postnikov
+-- Dr. Andrei Postnikov Tel. +33-387315873 - mobile +33-666784053 ---+
| Paul Verlaine University - Institute de Physique Electronique et Chimie, |
| Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France |
+-- [EMAIL PROTECTED] http://www.home.uni-osnabrueck.de/apostnik/
--+
