Isolated system Geometry Optimizations

[Babgen Manookian]

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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