  We compare definitions of the internal energy of an open quantum system and
strategies to split the internal energy into work and heat contributions as
given by four different approaches from autonomous system framework. Our
discussion focuses on methods that allow for arbitrary environments (not just
heat baths) and driving by a quantum mechanical system. As a simple application
we consider an atom as the system of interest and an oscillator field mode as
the environment. Three different types of coupling are analyzed. We discuss
ambiguities in the definitions and highlight differences that appear if one
aims at constructing environments that act as pure heat or work reservoirs.
Further, we identify different sources of work (e.g. coherence, correlations,
or frequency offset), depending on the underlying framework. Finally, we give
arguments to favour the approach based on minimal dissipation.
