Explicit differential equation based model of protein folding
What you are looking for is a force field: http://en.wikipedia.org/wiki/Force_field_(chemistry)

Standard force fields for proteins include CHARRM and AMBER. These have relatively simple, well-defined expressions for defining the various types of interactions between bonded and non-bonded atoms. Parameters are available for all standard atom types, and are fit to experimental and/or electronic structure calculations. These force fields are considered "classical" in that they do not explicitly treat electronic interactions, but rather try to capture these interactions in a coarse-grained way. Typically, there are expressions for describing the following interactions:


bonded interactions (bond stretching)
angle interactions (bond bending)
torsion interactions (dihedral bending)
12-6 Lennard-Jones interactions (non-bonded, dispersion forces)
Short-range charge-charge interactions (Coulombic interactions between charged or partially charged atoms)
Long-range charge-charge interactions (see Ewald summation method for more details)


Note that in these classical models, partial charges are generally statically assigned to each atom. The functional form of these different terms varies depending on the force field and sometimes implementation. These interactions are typically pairwise additive, so to get the total potential energy of the system you simply add together all the various interactions between the appropriate pairs of atoms.

There are certainly more rigorous models that attempt to capture other electronic structure affects, however for large molecules such as proteins, relatively simple force fields are generally applied. 

Please note that finding the global minimum in potential energy of a protein is not a trivial problem. People have been working on this for years, it's extremely difficult to develop algorithms that do not get trapped in local minima.