The ABINIT Users Mailing List ( CLOSED )

[Damien Caliste <damien.caliste@cea.fr>]

Hello,

Le 06/06/2007, "Davide Sangalli" <davide.sangalli@gmail.com> a écrit :
>     I'm studying the N2+ molecule within DFT/TDDFT.
> I founded out that, increasing the super-cell dimension
> (i.e. increasing acell) the total energy of the system is
> increasing (decreasing in module)
Yes, it is normal, it is an artifact due to the supercell approximation.
Your system is treated with periodicity, especially in the computation
of the Hartree potential and of the kinetic operator. The attached
curves show that the energy converges slowly with respect to the
super-cell size and that the exact isolated treatment of the Hartree
potential does not help (for the two cut-off energies, the periodic
Hartree treatment and the isolated one are superimposed for the
neutral silane molecule). The major responsible source for this behavior
(for neutral molecule) is then the kinetic operator.

> Am I getting this because N2+ is a charged molecule?
Yes, the fact that your system is charged does not help, because of
dipolar interactions in the Hartree potential because of the supercell.

The treatment of Hartree can exactly be done in isolated conditions
using the input variable icoultrtmt to 1 (see the quick documentation
on the web site
http://www.abinit.org/Infos_v5.3/input_variables/vargs.html#icoultrtmt).

Damien.

Attachment: energies.eps
Description: image/eps