Hello Vitor, GGA functionals typically do a poor job of capturing the physics of systems with strong van der Waals character like the Ar dimer. Typically LDA performs better than GGA because of fortunate cancellation of error, but is still far from correct.
Try installing a recent version of SIESTA's development trunk, available from the website. You will be able to use the vdW-DF (DRSLL), vdW-DF2 (LMKLL), and opt-b88 (KBM) functionals which may significantly improve your Ar-Ar bond distance and binding energy at the expense of additional time spent calculating the xc energy. There is a decent pool of computational results in the literature to draw from here, as well. Furthermore, these self-consistent vdW functionals will give you the tools to engineer a better optimized basis for Ar. Using an optimized basis rather than the automatically-generated ones can be useful and it probably couldn't hurt to add additional zetas, polarization orbitals and diffuse orbitals to your basis. Without your .fdf file though, it's difficult to give you a more exact perscription, but using some form of vdW-DF could be a decent start, I feel. Best of luck, On Thu, Jun 27, 2013 at 2:28 PM, Vitor Damiao <[email protected]> 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 > -- *Abraham Hmiel* Katherine Belz Groves Fellow in Nanoscience Xue Group, College of Nanoscale Science and Engineering at SUNY Albany http://abehmiel.net/about
