Physics focuses on describing matter by its extrinsic behavior in systems with other entities of matter, or the systems that matter behaves in. Chemists and physicists are trained differently, and they have different professional roles, even when working in a team. The division between chemistry and physics melds at their interface where the extrinsic and intrinsic systems of matter combine, notably in such fields as physical chemistry, chemical physics, quantum mechanics, nuclear physics/chemistry, materials science, spectroscopy, solid state physics, solid-state chemistry, crystallography, and nanotechnology.