Geometry Optimisation of Large Charged Systems

[Matt W]

Hi Olivia,

Mg2+ is nearly pathological if you use the default q10 pseudo and the GPW 
method. It needs a plane wave cutoff of around 1200 Ry or so to start 
getting reasonable answers. If you are sticking with non-hybrid 
functionals, like PBE, then try using the GAPW method (QS/METHOD section) 
and see if that helps. A cutoff of around 350 Ry should give well converged 
numbers then.

Matt

On Thursday, July 16, 2015 at 11:41:44 AM UTC+1, Olivia Lynes wrote:
>
> 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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