The Gibbs free energy is given by , where "H" is the enthalpy, "T" is the absolute temperature, and "S" is the entropy. , where "U" is the internal energy, "p" is the pressure, and "V" is the volume. "G" is the most useful for processes involving a system at pressure "p" and temperature "T", because, in addition to subsuming any entropy change due merely to heat, a change in "G" also excludes the work needed to "make space for additional molecules" produced by various processes. Gibbs free energy change therefore equals work not associated with system expansion or compression, at constant temperature and pressure. (Hence its utility to solution-phase chemists, including biochemists.)