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

[Nicola Marzari]

My guess is that if you make your cell larger and larger the difference
will, very slowly, disappear.

You have two sources of error

1) the "covalent" interaction between CO2 and H. You do not see this
in Gaussian because of the shortcomings of the basis set there -
there are no states able to represent the weak overlap of wavefunctions
midway between the molecule and the atom.

2) the dipole of the system, that interacts with itself periodically 
repeated. See e.g. Makov Payne PRB 1995, or cond-mat/0602599 and Refs. 
[14] in it.

Let us know if 1) or 2) are to blame.

			nicola



Xunlei Ding wrote:

> 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
> 
> 
> 
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Prof Nicola Marzari   Department of Materials Science and Engineering
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