There are two lines of thought that lead to the thesis that the subject matter of chemistry is dependent in some way on the subject matter of physics. Each connects the thesis directly with its main source of empirical support, the sciences, and in particular their methods, their theories and inter-relations among them. The first is mereological: properties of wholes depend in some way on (and may even be "nothing more than") the properties of their parts. If physics studies the parts of the kinds of things that chemistry studies, the dependence claim follows. And this makes physics basic to the explanatory aims of chemistry itself, for chemistry, so the argument goes, is explanatorily analytical: to explain what things do, it looks to their parts. The second line of thought is that the science of physics aims at full coverage. Physical laws cover everything, but the laws of other sciences, including chemistry, are of restricted scope: their truth does not require their full generality. Since physical laws cover everything, including chemical systems and their parts, if possession of a chemical property confers genuine causal powers, this must be in virtue of some relationship that that chemical property bears to some property that falls under a physical law. But the mereological claim has bite only if determination can only flow upwards that is, if the completeness claim is true. Hence this paper is concerned with this second line of thought. In what follows I will first explore the role of the completeness claim in contemporary physicalism, and then examine some of the arguments offered in its support. A good way to examine a thesis is to consider a position which denies it. Hence emergentism which is committed to the existence of downward causation will occupy Section 2. In Section 3 I will apply C. D. Broad's characterization of downward causation to quantum chemistry, arguing that the molecular Hamiltonians standardly cited in spectroscopic explanations fit Broad's account of physical explanation rather than the physicalists'.