[CPMD-list] PW91 implementation

[Oleksandr Shepelyev]

Dear CPMD Users!

I want to implement the PW91 (GGA) density functional into CPMD.
I want to use Vanderbilt PW91 pseudo-potentials. I did not find
a PBE pseudo-potentials for my atoms. So I thought that I can use
PBE part of code in lsd_func.F as an example. The Perdew-Wang 1991
GGA Correlation Functionals are the same in PBE and PW91. The PBE
Exchange Functional and The Perdew-Wang 1991 GGA Exchange Functional
are different in potential F(S) only.

But I found that the description of potential F(S) in lsd_func.F is
slightly different from the PBE functional described by John Perdew.
In the PhysRevLett F(S)=1+R-R*(1+m*S^2/R)^-1
But in code the first constant 1 is omitted: F(S)=R-R*(1+m*S^2/R)^-1

Also it is not easy for me to understand how the partial derivatives
V1X and V2X was obtained. I found in the code that V1X=dE/drho and
V2X=1/grad(rho) * dE/grad(rho), but...

Any help is much appreciated!

Sincerely yours,

Alex Shepelyev