How can I perform a SIESTA calculation using VdW functional?
Should the pseudopotential (.psf) be generated with ATOM with the desired flavour of VdW functional?
It is possible to perform a simulation using a VdW functional in SIESTA using the flags
XC.functional VdW
The other flag, XC.author, it is used specify the flavour of the VdW functional accordingly to what there is written in the manual.
Regarding the generation of the pseudo (.psf), it is not recommended to generate the pseudos with VdW functionals. We suggest to generate the pseudopotentials with a GGA, optionally one can change the exchange part by that of the vdW functional to be used.
This is because due to the pseudopotential approximation with van der Waals functionals the influence of the core can extend beyond the nominal “pseudopotential cutoff”, leading to pseudos that can, for example, converge to the Coulomb tail too far out.
Because of the non-locality of the VDW functional, a pseudo wavefunction that coincides with the all-electron wave function beyond a typical psp cutoff, does not give a pseudo potential that recovers -Z/r close to that cutoff, it can go smoothly over a long distance before PSP(r) - Z/r ->; 0. The “core size” becomes very large and that is problematic.