VASP Forces. How are they calculated? Where to look?

[vasilii]

Dear VASP Community,

for academic purposes I want to figure out the expressions of how the forces, acting on atoms are calculated.
In OUTCAR file I see the following terms: electron-ion (+dipol), ewald-force, non-local-force, convergence-correction.
What are the explicit expressions for them and where am I suppose to find it?
Or should I reformulate the question: is this information available for users? Or it's not because of commercial policy?

The only information I succeed to find was on this page:
https://cms.mpi.univie.ac.at/vasp/vasp/Forces.html
According to that, the forces are defined as partial derivatives over atomic coordinates. But there is no any hint on how are they calculated. Could it mean that the forces are calculated numerically by means of finite difference method?

Sorry for nooby questions, but I really need help with that.

Thank you in advance,
VD

[merzuk.kaltak]

The explicit expressions of the forces are written In Georg Kesse's PhD thesis "Ab-initio Molekular Dynamik für Metalle".
The thesis is available in the library of the Technical University of Vienna.