Geometry Optimisation of Large Charged Systems

Olivia Lynes livl... at gmail.com
Fri Jul 17 14:44:26 CEST 2015


Hi Matt,

Thanks for that info. Is this just a problem that's specific to Mg? I'll 
see what happens with the GAPW method, do you know if it's much more 
expensive than the standard GPW method?

Thanks

Olivia

On Thursday, July 16, 2015 at 9:48:22 PM UTC+1, Matt W wrote:
>
> 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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