I don't know if this is really applicable but I want to add a custom force (say, $F(x,y,z)$) to some atoms in my geometry optimization. Is there any way to add such forces to the DFT or molecular mechanics calculations? (preferably GAMESS or Gaussian for QC or Avogadro for MM).
It can be also done as Molecular Mechanics (MM) as I just need an approximate geometry after applying the force.
Let me give an example. Say I have molecule A and molecule B and a force that changes in space as $F(x,y,z)$ but is only applied to molecule B. Now I want to calculate the new geometry of A and B.
I can think this is like adding a $V_F$ to the external energy in the Schrödinger equation, but I don't know how to do that in QC packages:
$$\left[-\frac{1}{2}∇^2 + V(r) +\color{red}{V_F(r)}\right]ψ(r) = Eψ(r)$$
Update: The idea of making a unit cell and making shrinkage on it looks great (suggested by Geoff Hutchinson here), but the problem is I couldn't figure out how to install CP2K on MacOS and also I am looking for something with a simple interface like Avogadro. I even tried Maestro by Schrödinger software, but couldn't make unit cell shrinkage for that.