Hi, Seasons greetings to everyone :-) I've been revising statistical mechanics and have stumbled across an area that I've always been a little 'hazy' on. By the term 'single-particle' state, is it meant that this is a particular quantum state that one (or more) particle(s) can occupy, a term which holds true regardless of whether the wave-function describing the whole system is separable? My understanding, quite possibly incorrect, is that single-particle states are the quantum states that each individual particle occupy (of course, it may be that more than one individual particle occupies the same quantum state), regardless of whether the wave-function describing the system can be expressed as a product of the wave-functions describing each single-particle state, or not. In cases where it can be, it is then possible to express the energy eigenvalues of the microstates of the system as a sum of the energy eigenstates corresponding to each single-particle state. Please enlighten me if this is incorrect. Many thanks.