Hi, In principle, plane-wave (PW) and atom centered orbital (AO) based codes should give the same results for physically observable energy differences as, e.g., cohesive energies, surface formation energies, adsorption energies, ect. Prerequisite is ofcourse that the same (or at least comparable) DFT functionals and Pseudopotentials are used, that the basis sets (PW as well as AO) are sufficiently converged, and all other parameters (k-points...) are converged.
Note that the Boys-Bernardi Counterpoise correction (BSSE) is a good test if the AO basis set is sufficient for adsorption energies. Only if the CP-correction is small (<10% of the adsorption energy), the BSSE-corrected energies are close to the converged results. Otherwise, things may be erratic. Also note that pyhsisortion of alkanes on TiO2 is dominated by van-der-Waals type interactions, which are not included in standard DFTs. In particular in such weakly interacting situations, the BSSE may become tricky and geometry optimizations not taking care of BSSE may lead to geometries, where after BSSE correction the ineraction enery is repulsive. Best regards, Ulrich. ----- Original Message ----- From: Giacomo Giorgi [mailto:[email protected]] To: [email protected] Sent: Fri, 16 Nov 2012 05:11:59 +0100 Subject: [SIESTA-L] Surface Formation Energy and Adsorption Energy > Dear all, > I am performing calculations of anchoring mechanism of small > organic molecules on top of TiO2 surface. > I get that the Surface Formation Energy of every single surface is > different from theoretical results obtained at Plane Waves level but the > trend (the difference between the Surface Formation Energy of the various > reconstructions) agrees very well with those PW calculated results. I know > this is the expected behavior, and I am pretty satisfied of the fact that > such relative differences are in extremely good agreement with those PW > calculated. > > Now, I now "attach" these small organic molecules on top of the surface > and similarly I find that the Adsorption Energy of the single mechanism is > far from that calculated at PW level, BUT here too the relative > differences > between various attacks, say the DeltaE(bidentate-monodentate), > DeltaE(monodentate-molecularly adsorbed) is in very good agreement with > the previously calculated at PW level (PWscf). > I wonder if this is the expectable behaviour. > Mind that I am calculating the BSSE corrections for all the cases. > Should the Adsorption Energy for each single attack be different from that > calculated at PW level? > > Second brief question. Can I say that BSSE is negligible (almost) when > studying small cluster reactivity (for example, butane or propane reacting > with a metal oxide catalyst)? > > Last...I am performing some NEB calculations with postprocessing tools > using SIESTA as geometry optimizer. Has anyone got experience with NEB > calculations? Could you in case suggest me a "safe" value for the > MeshCutoff? 100-120Ry could be a reasonable value? > > > Thanks a lot for the attention! > > Very best regards, > Giacomo > > -- > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * > * Giacomo Giorgi * > * Assistant Professor * > * Research Center for Advanced Science and Technology (RCAST), * > * The University of Tokyo * > * 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan * > * tel./fax: +81 3 5452 5442 * > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * > > > > > > > "Oltre le illusioni di Timbuctu' > e le gambe lunghe di Babalu' c'era questa strada... > Questa strada zitta che vola via come una farfalla, > una nostalgia, nostalgia al gusto di curacao... > Forse un giorno meglio mi spieghero'..." > > (Paolo Conte, "Hemingway") > > --- Max-Planck-Institut für Eisenforschung GmbH Max-Planck-Straße 1 D-40237 Düsseldorf Handelsregister B 2533 Amtsgericht Düsseldorf Geschäftsführung Prof. Dr. Jörg Neugebauer Prof. Dr. Dierk Raabe Prof. Dr. Martin Stratmann Dr. Kai de Weldige Ust.-Id.-Nr.: DE 11 93 58 514 Steuernummer: 105 5891 1000 -------------------------------------------------
