Total Energy oscillates during optimization

[nadler]

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. Thanks a lot!

Regards,
Roger
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