Binding energy of graphene bilayer: CHARMM vs. DFT-D3

[Robert Yang]

Dear all,

I am calculating the binding energy of graphene bilayer of the size 3nmX3nm 
(2X416 atoms). It turns out that the result from calculations with 
classical force field is quite different from that of DFTs:

CHARMM FF:                                       -0.579 Hartree
DFT-D3:                                                -1.056 Hartree
DFT-D3 with BSSE correction:           -0.968 Hartree

It is quite strange that the value of CHARMM is nearly one half of DFT 
results. 

The binding energy is defined here as: dE = E_bilayer - 2*E_single_layer.

All my input files are attached in the tar file. Please help me to find out 
where the problem lies in. Thank you!

Best wishes,
Robert
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