Total Energy oscillates during optimization

[Axel]

On Thursday, October 20, 2011 9:51:54 AM UTC-4, nadler wrote:
>
> Dear all,
> I am performing a geometry optimization of a pyrite nanocrystal with one 
> phosphine oxide ligand adsorbed. The problem is that the "Total Energy" 
> starts to oscillate after a while and I have no idea why. It happens with 
> PBC set as well as without PBC. Also, when the pyrite structure is allowed 
> to relax or without ligand the oscillation occurs and the optimization takes 
> quite a long time. With the same input I can successfully optimize different 
> structures like e.g. TiO2 without such oscillations. I attach a graph of the 
> Total Energy plotted vs. step number and the input. 
>
> Does anybody know what the reason could be for the oscillation and how I 
> can get rid of it? Let me know if more information is required.
>

my first guess would be that your density cutoff is insufficient for the 
iron pseudopotential,
i.e. the density is not well represented and thus leading to jumps in forces 
and energies
whenever the electron density near them changes. 280 ry is likely not 
sufficient
for a semi-core iron pseudopotential.
 
cheers,
    axel.

Thanks a lot!
>
> Regards,
> Roger
>
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