Isolated system Geometry Optimizations

[Babgen Manookian]

Hey Juerg,

Thank you so much for the reply. I re-converged my cutoff values for the 
larger cell size and found that 400Ry and 80Ry produce energies which have 
much less deviation. What I do notice is that there is still the 
oscillation of the energy as the cell size changes. Is there a reason for 
this oscillation occurring?

My thoughts on this are that the planewaves are being cut at the cell wall 
at different values as the cell size changes which in turn causes the 
energy to oscillate. I would imagine if I could force the calculation to 
set the density equal to zero at the cell wall then there would be no 
oscillation. I am not sure if this reasoning is correct.

Bobby 



On Tuesday, February 14, 2017 at 1:21:43 PM UTC-5, Babgen Manookian wrote:
>
> Hey Everyone,
>
> I am a new cp2k user and have been working on a test system in order to 
> get a feel for how the program works. My test system is a cyclohexane 
> molecule and I am interested in determining the energy difference between 
> boat and chair conformations. I am using a wavelet poisson solver with 
> periodicity turned off and the molecule centered in the cell. I attached 
> the template input file which I used to create the different inputs with 
> varying cells size.
>
> What I have noticed is that as I increase my cell size the energy does not 
> converge on a single value, instead it seems to oscillate. The graphs below 
> show the energies of each conformation and their differences as functions 
> of cell size. I am very curious as to what is causing these oscillations. 
> If I have a single cyclohexane molecule centered in my cell, as I increase 
> my cell size, I would think that the extra vacuum space around the molecule 
> will not affect the energy. 
>
> If there is anyone out there who could provide any sort of input on this 
> it would be much appreciated.
>
> Thanks,
> Bobby
>
>
> <https://lh3.googleusercontent.com/-XcaU03VKFW0/WKNIuiBUTMI/AAAAAAAAATM/fJ2NHXSj-iswt7iCpMupCDoEpmOO9tarwCLcB/s1600/cychex_graphs.jpg>
>
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