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Specifically, in the General Utility Lattice Program (GULP) 2003 paper, in the methods section on Two-body Short-range Interactions we have the following:

For covalently bonded atoms, it is often preferable to Coulomb subtract the interaction and to describe it with either a harmonic or Morse potential.

What exactly does that mean? Does that mean the Coulomb interaction has no effect at all when using a pair potential? That seems to be what it's implying to me, but I can't find an explanation of the terminology anywhere.

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Got a bit too long for a comment, so I'll leave it as a semi-complete answer for now. In case the paper is inaccessible to other readers, much of the same information is in the gulp5.2 manual.

In regards to the question, it does seem to elaborate a little in the sentences following your quote. From earlier in the document:

Here the Coulomb interaction, and usually the dispersion one too, is subtracted for interactions which are between neighbours (i.e. bonded or 1-2) and next nearest neighbours (i.e. which have a common bonded atom, 1-3) according to the connectivity. This is done so that the parameters in the two- and three-body potentials can be directly equated with experimentally observable quantities, such as force constants from spectroscopy.

I think it essentially just amounts to a shift of the equations in order to make the parameters of the potential corresponding to physically meaningful/experimentally measurable quantities.

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  • $\begingroup$ Awesome! So do you think Coulomb subtraction means "Don't do the interaction" and is just unfortunately named? Or do you think the program does all the interactions and then literally subtracts the relevant ones? $\endgroup$
    – Connor
    Commented Oct 12, 2021 at 8:37

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