[CP2K:6784] Re: Geometry Optimisation of Large Charged Systems

[S Ling]

Hi

Following what Matt has said, i.e. you might need to use an extremely large 
cutoff for the q10 pseudoptential of Mg, if you are happy with the 
GTH-PBE-q2 pseudopotential of Mg with only two valence electrons for your 
system, you may try the attached DZVP-q2 basis set which I have reoptimised 
for Mg to be used with the GTH-PBE-q2 pseudopotential. You should be able 
to get reasonable results with a normal cutoff (e.g. ~400 Ry) using the q2 
pseudopotential of Mg.

SL


On 16 July 2015 at 21:48, Matt W <MattWa... at gmail.com> 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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Mg DZVP-q2
 1
 2 0 1 4 2 1
      1.45920927  0.09820758 -0.11426675 -0.05305226
      0.66519165 -0.07142564 -0.23288441  0.07892509
      0.11168453  0.10631928  0.95961265  0.98269301
      0.03989023  0.98688898 -0.10886471  0.15896770