[Chimera-users] APBS maps

Hey guys, i'm working with some electrostatic potential maps and am interested in doing a difference map. Two nearly homologous proteins with nearly identical surface shapes, and radically different electrostatics. I"m thinking a difference map would illustrate the differences well, but in looking at the APBS output I have no idea how to do that manually. Does anyone here have an idea about how to do that in chimera?