These are different, but related. issues: BSSE is an error that comes from the incompleteness of atom-centred basis sets. Basically, if you add atoms, you add basis functions. If you move atoms closer together, they can use each other's basis functions, which "improves" the basis. The result is an artificial lowering of energies for compact systems, and therefore an attractive force. You can fix this by using counterpoise correction, i.e. placing "ghost atoms" (basis functions without electrons or nuclei) where the atoms would be. You can use this in siesta for energies, but not forces or geometry optimisation. Just use the negative atomic number (i.e. -6 for a "ghost C").
Van-der-Waals forces are mainly based on dispersion, which is neglected or badly described by most DFT functionals. You can add an empirical force field term, based on work by Grimme. This is implemented in Siesta 3.0 (see section 6.20 of the manual). And finally, long-range interactions need diffuse basis functions. This also reduces BSSE (but doesn't remove it completely, and won't give you dispersion). You find an application example in Phys Rev B 79, 075441 (2009). In practice, you have to optimise basis functions. There is an example script in the siesta distribution, but it's tricky. HTH, Herbert Quoting Juzar Thingna <[email protected]>: > Hello Everyone, > > I'm working with vdw interractions for C in a double wall carbon nano-tube. > What I have realized is that using the vdw requires very good basis set and > the regular DZP doesn't work anymore. Even TZDP is not sufficient, according > to Guillermo Roma´n-Pe´rez et al. PRL, 103, 096102 (2009), they needed a > TZDP optimized basis + BSSE corrections. > Please could some explain of how to actually apply BSSE corrections? > Also I read some discussions on the previous threads about using diffuse > orbitals instead of BSSE, how do i use diffuse orbitals in siesta? > > If both the above options can be used I would like to know which one is > computationally more efficient. > > P.S. If possible please could you provide examples. > > Regards, > > Juzar Thingna > Department of Physics, > Center for Computation Science and Engineering, > National University of Singapore. > -- 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 ------------------------------------------------------------------ University of St Andrews Webmail: https://webmail.st-andrews.ac.uk
