In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol G ) is a thermodynamic potential that can be used to calculate the maximum amount of work, other than pressure-volume work, that may be performed by a thermodynamically closed system at constant temperature and pressure. It also provides a necessary condition for processes such as chemical reactions that may occur under these conditions. The Gibbs free energy is expressed as G ( p , T ) = U + p V − − T S = H − − T S Where: