Upon another look at your units, I'm confident that the counterpoise correction will get you much closer to the accepted energy value. For Ar-Ar ~4 Angstroms apart, the amount of the basis set correction will probably be around ~0.02 eV. For more info, see this, except instead of a molecule and a surface you have a molecule and a molecule: http://www.mail-archive.com/[email protected]/msg02916.html
On Thu, Jun 27, 2013 at 9:40 PM, Abraham Hmiel <[email protected]> wrote: > Vitor, > > Although I agree that you will have to perform a counterpoise correction > on the energy, that will only lower the binding energy by about 0.1-0.2 eV. > So, something else is wrong here. > > I took another look at the original Grimme paper and found that you're > using too high of a Grimme potential parameter. Supposing C6 for Argon is > 4.61 (Table 1) from the paper J. Comput. Chem. Vol 27, 1787-1799 (2006), > the geometric average of C6 for two Argon atoms (eq. 13) is, unsuprisingly, > 4.61 and that is the value you should use in your MM.Potentials Block, > rather than 47.85. > > There is a parameter, MM.Grimme.S6, missing in your input file. Experience > suggests that the default value for this parameter is usually not good > enough to capture the physics of the vdW interaction. > > As Herbert said, plotting Ar-Ar distance versus the binding energy will > clearly show you the trend (you will have to perform a ghost atom > calculation for each distance, look elsewhere on the list for help with > that, I believe I answered a similar question some time ago). You should > see that the PBE functional does not bind Ar-Ar together, with no minimum > in Binding E vs distance, and the DFT-D+PBE scheme will probably bind it > slightly. > > Increasing your basis set size is going to play a big role in how your > calculation comes out. If you don't want to use the simplex method to > optimize your basis, try out TZDP or a similar, larger basis. You must > declare your own basis set if you wish to include diffuse orbitals (s-like > orbitals of higher principal quantum number). > > Cheers! > > > On Thu, Jun 27, 2013 at 8:25 PM, Vitor Damiao <[email protected]>wrote: > >> Hi Abraham, >> >> follows the file .fdf to see if you can help me. >> >> argon dimer calculations showed acceptable results with the vasp and g03 >> programs. >> >> thanks, >> Vitor. >> >> >> 2013/6/27 Abraham Hmiel <[email protected]> >> >>> 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 >>> >>> >> > > > -- > *Abraham Hmiel* > Katherine Belz Groves Fellow in Nanoscience > Xue Group, College of Nanoscale Science and Engineering at SUNY Albany > http://abehmiel.net/about > > -- *Abraham Hmiel* Katherine Belz Groves Fellow in Nanoscience Xue Group, College of Nanoscale Science and Engineering at SUNY Albany http://abehmiel.net/about
