Dear Vitor,
You are encountering BSSE (basis set superposition error). Google it and
you will find some papers explaining it in detail. Basically, any method
using atom-centred basis sets artificially overstabilises compact
geometries. The standard way of handling it is called counterpoise
correction, and you can use it in SIESTA using ghost atoms (for
single-point energies; few programs allow you to use it for
optimisation). In the case of atomic dimers (such as yours) it would be
straight forward to scan the distance using counterpoise corrected
energies. What you should find for GGA functionals like PBE is no
attraction at all without VdW correction (see J. Chem. Phys. 116, 9620
(2002)). The DFT+D correction will help, but I'd be surprised if it got
systems as fickle as Ar2 right.
HTH,
Herbert
On 27/06/2013 19:28, Vitor Damiao wrote:
Hi all,
Have anybody calculated the argon dimer with SIESTA program? It appears
that the binding energy (BE) is overestimated. Please, see a DFT-PBE
calculation using a DZP basis set for Ar---Ar:
BE (DFT) = 1.3 kcal/mol
BE (DFT+D) = 2.4 kcal/mol
BE (Expt.) = 0.28 kcal/mol
I have tried using different pseudopotentials schemes, but the binding
energies appears to be too high. I would be grateful if someone could
explain me this trouble.
Thanks in advance
Vitor
--
Herbert Fruchtl
Senior Scientific Computing Officer
School of Chemistry, School of Mathematics and Statistics
University of St Andrews
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No SC013532
