Comparison with Gaussian DFT calculation

Babgen Manookian manoo... at gmail.com
Thu Jan 26 18:42:03 CET 2017


Hey Matt,

Thank you for the answer. I am working on running the calculations with 
bigger cell size at the moment. In the meantime, I am wondering if there is 
a way to turn off the periodicity in CP2K. I tried using the keyword 
PERIODIC NONE but got the exact same results.

Any help will be much appreciated it.

Thank you,
Bobby

On Monday, January 23, 2017 at 4:59:51 PM UTC-5, Matt W wrote:
>
> Hi,
>
> it looks like you cell is very small. Unlike Gaussian CP2K tends to run 
> with atoms periodically repeated inside a box. Your's is about 5x5x5 
> Angstrom^3. Probably you have some steric clashes between images of the 
> molecule, and likely these are different between the conformers.
>
> Matt
>
> On Monday, January 23, 2017 at 8:05:24 PM UTC, Babgen Manookian wrote:
>>
>> Hello all,
>>
>> I am new to CP2K and am working on making the transfer from Gaussian09. I 
>> am trying to match energy calculations between the two programs but am 
>> having trouble so I hope someone can help me.
>>
>> System: Just for testing purposes, I want to calculate the energy 
>> difference between chair and boat conformations for a cyclohexane molecule.
>>
>> Gaussian09: for my Gaussian calculations I am using DFT with BLYP 
>> functional and a 6-31G* basis set. Using this setup I obtain an energy 
>> difference of ~34 kJ/mol which is in fair agreement with a textbook value 
>> of ~30 kJ/mol.
>>
>> CP2K: For CP2K I am using DZVP-GTH-BLYP basis sets and GTH-BLYP 
>> pseudopotentials for both carbon and hydrogen atoms. Pretty much all other 
>> input parameters are set to default values or in accordance to the ones 
>> given in the Si test files found on the CP2K website. The input file is 
>> attached for further detail if needed. With this set-up I am obtaining an 
>> energy difference of ~288 kJ/mol. Clearly this value is way off the 
>> expected value and my Gaussian-obtained value and I am totally lost as to 
>> why.
>>
>> I have done these calculations with different values for cutoffs and 
>> different basis sets and in all cases I get a similar result where the 
>> difference between the two methods is roughly ten-fold. I am wondering if 
>> there may be something trivial that I am doing that is causing this.
>>
>> Any sort of insight will be truly appreciated.
>>
>> Thank you very much in advance.
>>
>> Babgen Manookian
>>
>
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