where etotal, volume and pressure are all instantaneous values. This
equation is analogous to the thermodynamic equation:

H = U + pV

This definition does not use a reference state (state where H is
identically zero). However, for a system at zero temperature (all
velocities = 0) and zero pressure (dEpot/dV = 0), the enthalpy will be
equal to Epot, where Epot is the potential energy of the interatomic
potential. So I suppose you could use that as the reference state. There
are other choices for enthalpy that can be made that may be more
appropriate for specific applications.