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Alex Granovsky
gran@classic.chem.msu.su

May it be the case you double count dispersion contribution to DFT energy for dimer?

B97D functional includes version 2.0 DFT-D correction by default.

Firefly incorporates DFT-D correction into nuclear energy term
and as such correction is already included into E(SCF) i.e. E(B97D).  
This means that the binding energy is simply:

E(AB) - E(A) - E(B)

provided that BSSE contribution to energy is small.

Hope this helps.

Kind regards,
Alex Granovsky






On Tue Jun 3 '14 2:15am, pejer wrote
------------------------------------
>Thanks for your replies

>Data for monomer (E in A.U.)
>A (MP2) = 983.276751611
>B (MP2) = 1138.267403399

>A (B97D)= 988.849046924
>B (B97D)= 1144.270528052

>Data for dimer
>AB (MP2) = 2121.541532633 A.U. = -1.64556648000143 kcal/mol
>AB (B97D+DFTD)= 2133.621287051 A.U. = 76.94027262530280 (kcal/mol)
>