[Pw_forum] Why the energies are different when two molecules are calculated together and separate?

[Xunlei Ding]

Dear all,
In a supercell (15A^3), I calculated the energies of one CO2 molecule, 
and a H atom, separately.
I got the energies E(CO2) and E(H). Then I calculated the system 
contained both CO2 and H in the same supercell, and get the energy E(CO2+H).
It is surprising to me that E(CO2+H) is lower than E(CO2)+E(H) by about 
more than 0.1eV.
All calculations are used PBE functional and with Gamma point.
I have put the H atom from CO2 at different places and the distances are 
tested for 6A and 9A. The cutoff are tested for 24Ry and 32Ry. The 
supercell are tested for 15A^3 and 20A^3. All these tests give the 
similar results and the energies differences are still more than 0.1eV.

Then with Gaussian03, both with PBC (period boundary conditions) and 
without PBC (just free molecules), the results show that E(CO2+H) is 
very close to E(CO2)+E(H).

It is found that the HOMO of H atom is also the HOMO of the system 
H+CO2, but in PWSCF results, HOMO of (H+CO2) is lower than that in H 
atom by ahout 0.1eV, while in Gaussian03 calculation, the HOMO is almost 
the same in H+CO2 and H . This is the reason why the total energy of 
H+CO2 is lower than E(H) + E(CO2).

I want to know the reason for this energy difference.

Thank you!

Yours sincerely,
Ding