A quantitative measure of the favorability of a given reaction under these conditions is the change Δ G (sometimes written "delta G " or "d G ") in Gibbs free energy that is (or would be) caused by the reaction. As a necessary condition for the reaction to occur at constant temperature and pressure, Δ G must be smaller than the non-pressure-volume (non- pV, e.g. electrical) work, which is often equal to zero (then Δ G must be negative). Δ G equals the maximum amount of non- pV work that can be performed as a result of the chemical reaction for the case of a reversible process. If analysis indicates a positive Δ G for a reaction, then energy — in the form of electrical or other non- pV work — would have to be added to the reacting system for Δ G to be smaller than the non- pV work and make it possible for the reaction to occur.