Ok Abraham and Herbert I will calculate with counterpoise correction and try to find acceptable results. On the Grimme potential parameters, the issue is the unit of measurement: 4.61 J nm6 mol-1 = 47.85 eV A6.
Thanks for helping! 2013/6/27 Abraham Hmiel <[email protected]> > 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 > >
