Hi all againAs I told you, I have a problem with pi-stacking distance, which is smaller than the typical experimental value .According with your suggestions I have solved my problem from two different ways: . a) I have reduced PAO.EnergyShift to 0.15 eV (before 0.30 eV), calculated pi-staking distance 3.548
b) I have used a DZ2P for S and C (my system has C, H and S) basis sets. However, I don't know it is well built. Calculated pi-staking distance 3.52
%block PAO.Basis C 2 # Species label, number of l-shells n=2 0 2 # n, l, Nzeta 0.0 0.0 1.000 1.000 n=2 1 2 P 2 # n, l, Nzeta, Polarization, NzetaPol 0.0 0.0 1.000 1.000 S 2 # Species label, number of l-shells n=3 0 2 # n, l, Nzeta 0.0 0.0 1.000 1.000 n=3 1 2 P 2 # n, l, Nzeta, Polarization, NzetaPol 0.0 0.0 1.000 1.000 %endblock PAO.BasisDoes DZ2P basis set well built? If so, which is the most correct solution? I think that a)........
Thanks Gregorio El 03/06/2010 15:47, Pablo A. Denis escribió:
Hi Gregorio,The BSSE is HUGE, and the results without BSSE are meaningless. It can be 0.5 eV for non bonded interactions. If you working with molecules you can do the BSSE optimization by hand or write a short script. For the benzene dimer the changes in the equilibrium distance when you optimize including BSSE are around 0.4 A.In some cases you may need to consider the monomer deformation energies.You can try to reduce the PAO.EnergyShift to 0.001 Ry, this helps a lot to reduce BSSE. I have tried to use a TZP basis set but as you said it was not useful, maybe you need a optimized basis set.Best Regards, Pablo----- Original Message ----- From: "Herbert Fruchtl" <herbert.fruc...@st-andrews.ac.uk>To: <siesta-l@uam.es> Sent: Thursday, June 03, 2010 8:44 AM Subject: Re: [SIESTA-L] BSSE and pi-stackingThe Van-der-Waals correction is an additional attractive force, correcting something that traditional GGAs don't describe. It lowers interaction energies and shortens bonds.BSSE is a spurious and unphysical attractive force. It also lowers interaction energies and shortens bonds. If you already have this error and add VdW correction, you will make things worse.We once did a quick study on the sandwiched benzene dimer, to find the "correct" S6 parameter (the scaling for the VdW interaction) for Siesta's default basis sets. We abandoned this, because the well-depth of the interaction was already massively overestimated through BSSE (I vaguely remember a factor of 8) compared to high-level literature results.I don't know a solution to this problem. Arguing that BSSE somehow simulates VdW interaction is of course dodgy. Besides the massive overestimation of its magnitude, it will have the wrong distance dependence. I don't know a solution to this problem, except increasing the size of the basis set, but I suspect it would have to be very big to remove BSSE. In the molecular case with Gaussian basis sets this is well studied, but I suspect that the limited range of basis functions in Siesta makes things worse.Herbert Ricardo Faccio wrote:Hi GregorioWhen you talk about Pi-stacking... do you mean Van der waals interactions?If this is the case remember the drawbacks of: LDA: cancellation of errors with a near overbinding tendency GGA: VdW interactions are totally neglectedPlease, consider the use of the new VdW xc-potential introduced in the newversoin of siesta. Regards Ricardo p.s.: it has nothing to do with BSSEOk, thanks. I tried with TZP basis set, but the results don't improve enough. Respect to use BSSE, I agree with you about the problem to use with my system, since the are two indentical molecules (interconvertible by symmetry) in the unit cell. Finally, I don't know how to include diffuse functions in SIESTA. Thanks Gregorio El 03/06/2010 12:49, Herbert Fruchtl escribió:If you mean counterpoise corrected geometry optimisation, that'stricky, and definitely not implemented in SIESTA. Counterpoise is onlyreally straight-forward for dimers (or at least small numbers) offairly rigid molecules. In a crystal, you would have to make sure thatyour supercell is big enough (you can't CP correct the interaction of a molecule with its image), and I don't know the implications for K-points (my first inclination would be to say if you need more than one K-point, your system is too dense for counterpoise to be meaningful). Have you tried adding diffuse functions? BSSE is a consequence of the incompleteness of localised basis sets. If you describe thewavefunction far away from the nuclei better, it should improve, but Idon't know how quickly. HTH, Herbert Gregorio García Moreno wrote:Hi all I am studing polymers pi-stacked. I have obtained good crystal parameters, except for the pi-stacking distance, which is smallerthan experimental data. Here, you recomended me that I should use TZPand BBSE. I have tried with TZP, but the results do not improve respect to DZP. However, I have searched in literature, and to apply BSSE could solucionate my problem. In the SIESA-list I have not found how to optimize with BSE, can someone help me? Thanks in advance Gregorio-- Herbert Fruchtl Senior Scientific Computing Officer School of Chemistry, School of Mathematics and Statistics University of St Andrews -- The University of St Andrews is a charity registered in Scotland:No SC013532
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