The ABINIT Users Mailing List ( CLOSED )

[Dan Ricinschi <ricinski@semi.ee.es.osaka-u.ac.jp>]

Hi!

I cannot confidently point out the cause of this behavior, but in my 
experience this happens when the optimization goes on for quite a 
large number of Broyden steps, without being able to find the 
absolute minimum of the total energy with the symmetry strictly 
enforced. It could also mean that the imposed symmetry is not that of 
the ground state, (although in many cases it could be possible to 
optimize the crystal under other desired symmetry). If you don't 
suspect a serious physical error, you could restart the optimization 
with the R3c structure obtained after adjusting those small 
deviations by hand and see what happens.

On the other hand, by looking at your atomic coordinates I think that 
it is likely that you have found a local minimum of the total energy 
surface, as the Bi atoms might be "fixed" to their centro-symmetrical 
positions. If you have not already done so, I suggest you to restart 
the optimization from scratch by displacing the Bi atoms slightly 
from their symmetrical positions, in the starting configuration. That 
is, use 0.01 0.01 0.01 and 0.51 0.51 0.51 instead of 0 0 0 and 0.5 
0.5 0.5. In this way the algorithm might find the optimized structure 
easier, without breaking the symmetry for lengths and angles.

I hope this helps.

Dan Ricinschi

At 06:34 PM 7/2/2007, you wrote:
> Dear abinit users,
>
> I have found a small inconsistency when doing geometry optimizations 
> of a rhombohedral (R3c) system.
> The angles and vectors stay the same (as given by the space group) 
> for about ten geometry iterations, and then one starts to diverge slightly.
> Within the limits of the space group, they should be the same.
> I guess, this is a problem of the floating-point precision?
> Below the worst case, I encountered so far, the first angle deviates 
> by 0.5 mdegrees from the two others.
>
>
>  Angles (23,13,12)=  5.96739156E+01  5.96739156E+01  5.96739156E+01 degrees
>   angles (23,13,12)=  59.67391564  59.67391564  59.67391564 degrees
>   angles (23,13,12)=  59.65429950  59.65429950  59.65429950 degrees
>   angles (23,13,12)=  59.60987717  59.60987717  59.60987717 degrees
>   angles (23,13,12)=  59.59774016  59.59774016  59.59774016 degrees
>   angles (23,13,12)=  59.58954710  59.58954710  59.58954710 degrees
>   angles (23,13,12)=  59.58994126  59.58994126  59.58994126 degrees
>   angles (23,13,12)=  59.59640191  59.59640191  59.59640191 degrees
>   angles (23,13,12)=  59.60556657  59.60556657  59.60556657 degrees
>   angles (23,13,12)=  59.61517982  59.61517982  59.61517982 degrees
>   angles (23,13,12)=  59.62547486  59.62547486  59.62547486 degrees
>   angles (23,13,12)=  59.64340038  59.64340037  59.64340037 degrees 
> ! start of divergence
>   angles (23,13,12)=  59.66163347  59.66163344  59.66163344 degrees
>   angles (23,13,12)=  59.67145659  59.67145646  59.67145646 degrees
>   angles (23,13,12)=  59.67329734  59.67329688  59.67329688 degrees
>   angles (23,13,12)=  59.67356472  59.67356266  59.67356266 degrees
>   angles (23,13,12)=  59.67416017  59.67414948  59.67414948 degrees
>   angles (23,13,12)=  59.67575367  59.67570578  59.67570578 degrees
>   angles (23,13,12)=  59.67768192  59.67750735  59.67750735 degrees
>   angles (23,13,12)=  59.67851421  59.67798716  59.67798716 degrees
>                            ^            ^            ^
>
>      acell    1.0086679784E+01  1.0086679784E+01  1.0086679784E+01 Bohr
>   acell=  1.0086679784E+01  1.0086679784E+01  1.0086679784E+01
>   acell=  1.0095845448E+01  1.0095845448E+01  1.0095845448E+01
>   acell=  1.0107077039E+01  1.0107077039E+01  1.0107077039E+01
>   acell=  1.0105341284E+01  1.0105341284E+01  1.0105341284E+01
>   acell=  1.0100122543E+01  1.0100122543E+01  1.0100122543E+01
>   acell=  1.0097132505E+01  1.0097132505E+01  1.0097132505E+01
>   acell=  1.0093418047E+01  1.0093418047E+01  1.0093418047E+01
>   acell=  1.0092339678E+01  1.0092339678E+01  1.0092339678E+01
>   acell=  1.0092419651E+01  1.0092419651E+01  1.0092419651E+01
>   acell=  1.0092238158E+01  1.0092238159E+01  1.0092238159E+01 ! 
> start of divergence
>   acell=  1.0090904084E+01  1.0090904084E+01  1.0090904084E+01
>   acell=  1.0088374760E+01  1.0088374763E+01  1.0088374763E+01
>   acell=  1.0086037737E+01  1.0086037745E+01  1.0086037745E+01
>   acell=  1.0084928271E+01  1.0084928299E+01  1.0084928299E+01
>   acell=  1.0084184900E+01  1.0084185025E+01  1.0084185025E+01
>   acell=  1.0083734213E+01  1.0083734862E+01  1.0083734862E+01
>   acell=  1.0084085342E+01  1.0084088246E+01  1.0084088246E+01
>   acell=  1.0084919226E+01  1.0084929815E+01  1.0084929815E+01
>   acell=  1.0085332361E+01  1.0085364332E+01  1.0085364332E+01
>      acell    1.0085332361E+01  1.0085364332E+01  1.0085364332E+01 Bohr
>                      ^                 ^                 ^
>
>
>
> The optimized atomic positions remain in the initially determined 
> space group, so if three equal angles and acell are given for a 
> subsequent scf, R3c is the space group obtained.
> (The same behaviour is also observed for optcell 3)
>
> BAO.in
> ------
> kptopt 1
> ngkpt 6 6 6
> ecut 45.0
> pawecutdg  60
>
> optcell 2
> ionmov 2
> dilatmx 1.05
> ecutsm 0.5
> ntime 25
>
> getwfk 1
>
> acell 10.086679784 10.086679784 10.086679784 bohr
> angdeg  59.67391564  59.67391564  59.67391564
> ntypat 3
> znucl 83 13 8
> natom 10
> typat 1 1 2 2 3 3 3 3 3 3
> xred
> 0.00000000000000    0.00000000000000    0.00000000000000
> 0.50000000000320    0.50000000000320    0.50000000000320
> 0.27141788893320    0.27141788893320    0.27141788893320
> 0.77141788893320    0.77141788893320    0.77141788893320
> 0.54462841053320    0.98371566192320    0.07314091094120
> 0.07314091094120    0.54462841053320    0.98371566192320
> 0.98371566192320    0.07314091094120    0.54462841053320
> 0.48371566192320    0.04462841053420    0.57314091094320
> 0.04462841053420    0.57314091094320    0.48371566192320
> 0.57314091094320    0.48371566192320    0.04462841053420
>
> nstep 100
> toldfe 1.0d-10
> diemac 12.0
> nsppol 1
> -----
>
> Which set of angles and vectors should be taken for subsequent scf-runs?
>
> Thank you very much for any hints.
>
> Best regards,
>
> Pio Baettig