reaction energies and box sizes

Geoffrey Wood ge.f... at
Mon Nov 17 22:58:11 UTC 2014

Hello everyone.

I'm a little bit confused as what to do with the energy convergence as a 
function of box size when computing reaction energies.

For example using the BLYP functional and the TZVP-MOLOPT-GTH basis set and 
a cut off of 1200 Ry (an example of the input deck that I used is attached) 
I computed the energy for the Cl atom (neutral) as a function of the box 
size. This converges at about 12 Angstroms but there are oscillations and 
the minimum actually  at 7 Angstroms, well before convergence (see attached 

My question for the community is if you are computing a  reaction energy, 

A  + B =  AB

do you 

a) use consistent Box sizes for all species?
b) use the minimum energy found as a function of box size for all three 
c) the converged energy as a function of box size (even though this may not 
be the minimum) for all three species?

Thanks, Geoff.

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