I'm a quantum chemist, and nearly every system I've ever cared about has been studied by DFT. However, I am currently trying to model the surface interactions on a nanoparticle around 100 nm in diameter. I know the composition of the nanoparticle, but I do not know which atoms are at the surface versus in the core. It's too large to really be reasonable in my regular DFT packages. Any ideas on where to start?