[Pw_forum] vdW correction : unexpected Binding energy

[Amita Sihag]

Dear all
I am trying to calculate the binding energy for DNA adsorption on 2D
sheets.I have done calculation by including vdW correction and simple PBE
calculation with Ultrasoft pseudopotential. While studying the adsorption
of the four nitrogen bases on the 2D surface I have observed that the
binding energy for the cytosine adsorption on surface is coming out to be
very different (almost three times larger) from what is expected when I
include the vdW correction (vdw_corr = 'DFT-D') in to the calculations.
Whereas, when I do only with PBE (no vdW correction) all the four bases
gives expected correct energies.The parameters which has been included for
vdW correction are as follows
      &SYSTEM
                       ibrav = 0,
                    celldm(1)= 1.89 ,
                         nat = 88,
                        ntyp = 5,
                    vdw_corr = 'DFT-D',
                      london = .TRUE.,
                     ecutwfc = 50 ,
                     ecutrho = 500 ,
                 occupations = 'smearing' ,
                    smearing = 'gaussian' ,
                     degauss = 0.020
/
&ELECTRONS
            electron_maxstep = 500 ,
                    conv_thr = 1.0d-5 ,
                 mixing_mode = 'plain' ,
                 mixing_beta = 0.7 ,
             diagonalization = 'david' ,
/
Please help me with this unexpected value of the binding energy value for
the cytosine? Any kind of help would be appreciated.




Thank you
Amita Sihag
Research Student, Dept. of Chemistry
Indian Institute of Technology Gandhinagar
Gujrat (India)
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