[Pw_forum] binding energy

[Moosavi Seyed Mohamad]

Hi

I calculated the binding energy of a cluster (species1) with a ligand(species 2) with this formula:
E_{total} - E_{species1} - E_{species2}

I run PAW in a cubic cell large enough to avoid periodicity effects in the calculations (10 angstrom each side).
The geometry converges very well to a 2.1 angstrom distance between species, as we expected.

However, the results are surprising from two respects.
First, the binding energy is too high. ~500 kcal/mol

Second, when I add new monomer to species2 (make a dimer), in principle, the binding energy should remain constant (The bond length remains the same).
However, I get extra ~200kcal/mol in the energy.

The number of electrons:
cluster (species1): 499
monomer (species2): 60
dimer (species2): 102

So, something should be wrong with the formula or a scaling factor is  needed here.
I am wondering if you can guide me through this.

Best regards
Mohamad


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