[CP2K-user] Binding Enery Curve for Si-S bond

[Matt W]

Hello, 

I just want to point out that the 'correct' answer depends on your level of 
theory - model chemistry Gaussian would call it.

In this case you are likely to do better to calculate the thermodynamic 
dissociation energy from the total molecule and the individual fragments. 
To get the dissociation curve can be even harder, you probably want a 
symmetry broken UKS solution and DFT really struggles. There is a very 
recent thread with similar comments from Matthias Krack on a not dissimilar 
system.

Matt

On Wednesday, October 17, 2018 at 7:34:37 PM UTC+1, Aniruddha Dive wrote:
>
> Hi All,
>
> I want to generate a binding energy curve for a Si-S bond with bond 
> lengths varying from 1.5 A - 7.5 A. To calculate the binding energy curve I 
> have generated a SiH3-SH molecule where I vary the Si-S bond lengths from 
> 1.5 A to 7.5 A over 25 different geometries. I run a geometry optimization 
> for each geometry keeping the Si and S atoms fixed while allowing other 
> atoms to relax. Based on the energies for all these geometries I obtain a 
> binding energy curve for Si-S which flattens out giving a bond dissociation 
> energy of 105 kcal/mol. The experimental Si-S bond dissociation energy 
> reported is ~ 145 kcal/mol which is significantly higher than what I 
> obtain. To generate the binding energy curve I take all the energies to be 
> relative to the minima which corresponds to zero energy.
>
> I am thinking that this difference could be due to the BSSE effects when 
> the Si-S atoms are at a larger separation leading to generation of two 
> fragments SiH3 and SH. To determine the BSSE, I ran a BSSE calculations for 
> the SiH3-SH molecule where the Si-S distance is 7.5 A. I obtain a 
> CP-corrected total energy from these calculatons. Now, while generating the 
> binding energy curve do I need to use the CP-corrected total energy as the 
> total energy for that geometry? Or should I subtract this CP-corrected 
> total energy from the total energy obtained during geometry optimization 
> which will give me the BSSE. Further should I add the BSSE obtained to the 
> relative energies for these geometries where the Si-S distance is larger 
> than 3.0 A.
>
> I have attached input file providing details about the GEO_OPT as well as 
> BSSE and the coordinate file for one case.
>
> Kindly let me know how can I generate an accurate binding energy curve for 
> these molecules?
>
> Thanks,
> Aniruddha M Dive
> PhD Candidate, Washington State University
>
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