[DFTB-Plus-User] Request for help understanding repulsive energy

[Benjamin Hourahine]

Hello Peter,

The DFTB electronic structure isn't influenced by the repulsive, other than through its effect on what the stable geometries will be.

I'm guessing, as you say, that these sources mean if you fit against data from a specific functional (LDA, GGA, B3LYP, ...), the DFTB energetics are then similar to that chosen functional. Assuming of course that it is a good parameterization for the case it's applied to.

Regards

Ben

       Dr. B. Hourahine, SUPA, Department of Physics,
     University of Strathclyde, John Anderson Building,
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________________________________
From: DFTB-Plus-User <dftb-plus-user-bounces at mailman.zfn.uni-bremen.de> on behalf of Peter Smith <petersmith2624 at gmail.com>
Sent: 21 May 2022 14:23
To: dftb-plus-user at mailman.zfn.uni-bremen.de <dftb-plus-user at mailman.zfn.uni-bremen.de>
Subject: [DFTB-Plus-User] Request for help understanding repulsive energy

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Dear,

I am planning on fitting my own set of SK-parameters to be used in DFTB+. However, I am confused about the repulsive energy term.

To my understanding, the repulsive energy, however it is fitted, does not influence the electronic behaviour (Mulliken charges, electronic energies etc.). It does not show up in the DFTB equivalent to the Kohn-Sham equation, as it falls out during the energy minimisation since it does not depend on the orbitals, but solely on the distance between types of atoms. That it does not show up here seems logical to me, as the terms that make up the repulsive energy also do not show up in the DFT Hamiltonian, as they are simply constants.
Still, some of the sources that I am using state that they consider "the repulsive energy as the practical equivalent to the xc-functional in DFT". Since the xc-functional does influence the electronic wave function, while according to my current understanding the repulsive energy does not, this statement makes me doubt my understanding.

I would understand it if they see the E_rep as an xc-functional equivalent due to the fact that it is simply used to correct some of the missing energy contributions to the total energy, just like the xc-functional. To make sure that it is not more than that, I am writing this email. Is my understanding correct that the repulsive energy does not influence the electronic behaviour (in a static calculation, i.e., if I am simply interested in calculating the partial charges on the atoms in my system)? And, in stead, that the repulsive energy is solely important when performing MD in DFTB+?

I guess this is a trivial question, but I simply want to make sure I understand and for this I am asking your help.

Kind regards,
Peter


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