[Pw_forum] discrepancy in molecular binding energy between pwscf and localized basis set program

[Paolo Giannozzi]

On Jun 11, 2007, at 20:22 , Garold Murdachaew wrote:
>
> [...] I did calculations of the binding energy of Xenon dimer
> (at the minimum separation of 4.42 Angstroms). I obtained (in meV):
>
> 	molpro	pwscf3.2
> LDA	-34	-34
> PBE	 -4	-11
>
> As can be seen, the LDA calculations agree perfectly but the PBE  
> pwscf calculation gives an interaction energy almost three times  
> more negative.

in French, there is a nice way to say that something is negligibly  
small:
"c'est trois fois rien" = "it is three times nothing". It perfectly  
describes
the above situation :-).

Seriously: gradient-corrected functionals are ill-defined in the
rho=>0 limit . In pwscf, they are cut when rho < small, defined
in Modules/functionals.f90. In molpro, I don't know. This is the
first thing I would check.

Paolo
---
Paolo Giannozzi, Democritos and University of Udine, Italy