Geometry Optimisation of Large Charged Systems

[Olivia Lynes]

Hi all,

I'm having a problem with a few of my geometry optimisations not converging 
after several thousand steps and having looked at the energy changes 
through the main output file it's not decreasing but is fluctuating a lot 
in a certain range. I'm at a loss to why as I've got similar calculations 
completed with similar input and just a different box size and they have 
converged without this fluctuation in a few hundred steps.

I've attached the input, coordination files and the first hundred or so 
steps of the output file.

These are geometry optimisations of an Mg 2+ ion in a box of 64 waters, 
with a cell size of 13.41 angstroms, so the overall system is charged 2. 
I've used the GTH DZVP basis sets and potentials. 

Having done a cell optimisation on just the 64 waters, we introduced the Mg 
ion and charged the system. In lieu of being able to do cell optimisations 
on a charged system, then geometry optimisations are being done at varying 
box sizes by 10ths of an angstrom as a sort of manual cell optimisation to 
try to find both a minimum energy structure and a minimum energy box size. 

I've done similar calculations using Ca and Sr and haven't had this issue 
with any of my jobs, they all converge in a few hundred steps. 

Is this potentially a problem using charged systems? Is it necessary for me 
to put a counter ion in which would let me do a standard cell optimisation? 

Or is this approach completely wrong and we would be better off doing MD? 

The aim is to do ab initio MD on these systems to look at the coordination 
of the ion in a bulk of water, so reaching a minimum energy structure and 
box size isn't essential but it would be nice to have the regular DFT data 
to back it up. 

Thanks in advance for any insights.

Olivia



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