[CPMD-list] combination of PBE USPP with BLYP DFT

[Jung Goo Lee]

Hi everyone,

I would like to know whether the combination of PBE uspp in ATOMS option 
and BLYP in DFT
option (the example of partial input is shown below)makes sense or not, 
especially, for calculations
of free energy (constraint). This is because BLYP method underestimate 
the dissociation free energy
compared with experiment. For example,  water dissociation in 32 water 
molecules with MT PP/
BLYP method gave 17 kcal/mol in free energy while experimental value is 
21 kcal/mol(Trout and
Parrinello, Chem. Phys. Lett, 288(1998)343.
I tried the same calculation with PBE uspp and BLYP method.
I got about 20 kcal/mol althogh I tried with  a few hundred fs (instead 
of 1 ps) MD at each point of 
O-H bond constraint. So, this combination (PBE uspp/BLYP) seems to be 
o.k. in free energy
calculations.

Is this a reasonable and cheap(computationally) way to get out of the 
problem with free energy
calculations with gbl uspp/BLYP?
Does any one have any rationalization or contradiction with reasonable 
explanation?

Jung-Goo Lee

Example
--
--
 &ATOMS
*008-O-gpbe--bm.uspp NEWF BINARY
  LMAX=P
  1
  4.3728  4.5000  4.5000       1   8

*001-H-gpbe--bm.uspp NEWF BINARY
  LMAX=S
  2
  5.5172  4.5000  5.9238       1   1
  5.5172  4.5000  3.0761       1   1
&END

&DFT
 OLDCODE
 FUNCTIONAL BLYP
 GRADIENT CORRECTION BECKE88 LYP
 LDA CORRELATION LYP
&END
--
--