We derive an alternative to the Wetterich-Morris-Ellwanger equation by means of the two-particle irreducible (2PI) effective action, exploiting the method of external sources due to Garbrecht and Millington. The latter allows the two-point source of the 2PI effective action to be associated consistently with the regulator of the renormalization group flow. We show that this procedure leads to a flow equation that differs from that obtained in the standard approach based on the average one-particle irreducible effective action.

6 pages, revtex format; amended to incorporate corrections from an erratum (submitted to PRD) that relate to the incorrect functional identity original reported for the inverse two-point function; conclusions otherwise unchanged