high binding energy

[Sandeep Kumar Reddy]

Dear all,
             I want to calculate binding energy and compare with Gaussian's
result and then i would like to proceed to bulk simulations.

My system one ion pair (it's an ionic liquid). I did geometry optimizations
of cation, anion and ion-pair and calculated the binding energy. This value
is equal to  *-119* *kcal/mol*. After adding BSSE corrections, it is
reduced to *-116* kcal/mol. For comparison, i carried out same calculations
at MP2/aug-cc-pVDZ level using Gaussian package and this value is equal to
-104 kcal/mol (BSSE corrected). A difference of 10 kcal/mol of such
calculation is discouraging.

(I have used same functional, cell parameters, cutoff, potentials for all
three calculations)


I repeated the calculations by changing cutoff (to 480 Ry) and increasing
box length (20 \AA). But it gives me same binding energy. Additionally, i
have done same calculations in plane wave basis set using CPMD code. It
gives 105 kcal/mol, very close to MP2 result.

I am not able to figure out why it happens.  Could anybody comment on my
input files ?


*Ion-pair :*

&GLOBAL
  PROJECT dimethyl_ammonium_bromide
  RUN_TYPE GEO_OPT
  PRINT_LEVEL MEDIUM
&END GLOBAL

&FORCE_EVAL
  METHOD Quickstep
  &DFT
    BASIS_SET_FILE_NAME  /panfs/home/Xsreddy/cp2k/tests/QS/BASIS_MOLOPT
    POTENTIAL_FILE_NAME  /panfs/home/Xsreddy/cp2k/tests/QS/POTENTIAL
    &MGRID
      CUTOFF 280
      NGRIDS 5
    &END MGRID
    &QS
      METHOD       GPW
      EPS_DEFAULT  1.0E-10
    &END QS
    &SCF
      MAX_SCF   50
      EPS_SCF   1.0E-6
      SCF_GUESS ATOMIC
      &OT ON
        MINIMIZER       CG
        PRECONDITIONER  FULL_ALL
        ENERGY_GAP      0.001
      &END OT
      &OUTER_SCF
        MAX_SCF  100
        EPS_SCF  1.0E-6
      &END OUTER_SCF
    &END SCF
    &XC
      &XC_FUNCTIONAL  BLYP
      &END XC_FUNCTIONAL
      &vdW_POTENTIAL
        DISPERSION_FUNCTIONAL PAIR_POTENTIAL
        &PAIR_POTENTIAL
          TYPE DFTD2
          REFERENCE_FUNCTIONAL BLYP
        &END PAIR_POTENTIAL
      &END vdW_POTENTIAL
    &END XC
    &POISSON
     POISSON_SOLVER WAVELET
     PERIODIC NONE
    &END POISSON
  &END DFT
  &SUBSYS
    &CELL
      ABC 20.0 20.0 20.0
      PERIODIC NONE
    &END CELL
    &COORD
    H       1.82622900    0.00011700    1.45609900
    H       0.41368700   -0.00011700    0.43855600
    N       1.53927100    0.00002300    0.47861600
    C       1.97625500   -1.24136700   -0.20041700
    H       3.06445100   -1.28419200   -0.25417500
    H       1.58040100   -2.09266400    0.34985900
    H       1.53907000   -1.23367800   -1.19754500
    C       1.97589400    1.24149000   -0.20039300
    H       1.54099200    1.23242900   -1.19850300
    H       1.57755100    2.09264100    0.34831700
    H       3.06414500    1.28582600   -0.25177600
    Br     -1.40269500   -0.00003600   -0.01818000
    &END COORD
    &TOPOLOGY
      &CENTER_COORDINATES
      &END
    &END
    &KIND H
      BASIS_SET  DZVP-MOLOPT-GTH
      POTENTIAL  GTH-BLYP-q1
    &END KIND
    &KIND C
      BASIS_SET  DZVP-MOLOPT-GTH
      POTENTIAL  GTH-BLYP-q4
    &END KIND
    &KIND N
      BASIS_SET  DZVP-MOLOPT-GTH
      POTENTIAL  GTH-BLYP-q5
    &END KIND
    &KIND Br
      BASIS_SET  DZVP-MOLOPT-SR-GTH
      POTENTIAL  GTH-BLYP-q7
    &END KIND
  &END SUBSYS
&END FORCE_EVAL
&MOTION
  &GEO_OPT
    MAX_FORCE 0.0001
    MAX_ITER 500
    OPTIMIZER BFGS
  &END GEO_OPT
&END MOTION


Other input files are enclosed.

Thanks.

Regards,
Sandeep


*
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