Is there a rule-of-thumb for selecting a cutoff for the Morse potential in molecular dynamics simulation? For instance, is it typically some multiple of the equilibrium distance?
Since you are referring to "the Morse potential", I assume you mean to use it for modeling van der Waals interactions?
If not, please ignore this answer and consider updating your question to include what kind of potential you are modeling using the Morse potential.
If so, here is my answer: There is a rule of thumb based on experience and a lot of previous studies, but not based on equilibrium distance as far as I am informed. Most of the times people use cutoff values of around 10-12Å nowadays. If you want to be rigorous you can test your cutoff with the system you want to study, it will greatly depend on the type of system you are studying. Check out the paper of Jablonka et al. for some insight into the problem of cutting the vdW potential. When necessary and available you might want to consider using tail corrections to smooth out the error introduced by cutting the potential. You will also find in there, that the influence of the cutoff on forces is however negligible. So if you are not analyzing energies but movement, the errors due to the cutoff are not that important.