Can I do grad studies in computational physics if my BSc is in chemistry?

I don't know what "developing functionals" is (I do construct functionals to extract data from probability distributions, but I have the feeling that is not what you mean) and I don't know the "grad school" system in the US. But I can tell you that our "Computational Biophysics" group has quite a few PhD students and post-docs who did their Masters or PhD in biology, chemistry, or engineering (although to be fair I must add that part of the reason for this is the lack of adequate physics applicants). There is a large overlap region between chemistry and physics (think soft condensed matter physics and physical chemistry), so I think that a lot of computational chemistry is sufficiently close to some computational physics to make a transition quite easy. But please keep in mind that this statement is merely based on knowing a bit about physics, chemistry, and computing - I don't know the US university administrations at all.