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

Lucas Fernandez Seivane quevedin at gmail.com
Wed Apr 12 19:45:18 CEST 2006

I believe that this is more or less the case that Prof. Marzari states
as 1). It may due to an effect well known related to localized basis
(LCAO, gaussians) which has to do with the number of wavefunctions in
each one of your simulations: the  Basis Set Superposition Error.
Quoting from http://iqc.udg.es/~perico/bbopt.html:
"The theoretical study of molecular interactions under the
supermolecular approach with finite basis sets centered at the atomic
positions originates the so-called Basis Set Superposition Error
(BSSE) Within the LCAO-MO approach, each fragment can be expanded to
some extent in the basis set of the partner. Thus, BSSE is the
unphysical effect due to the improvement of the quantum mechanical
description of the fragments within the supermolecule. It has been
recognised for long time that this effect results in an increase of
the interaction energy."

Some ideas for dealing with it are seen for instance in
http://arxiv.org/PS_cache/physics/pdf/9908/9908022.pdf or in

On 4/12/06, Nicola Marzari <marzari at mit.edu> wrote:
> 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
> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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