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

[Peter Smith]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.zfn.uni-bremen.de/pipermail/dftb-plus-user/attachments/20220521/4f633d40/attachment.htm>