WHAM is an extension of umbrella sampling provides a means of determining an unknown underlying potential from a set of simulations performed on a system with addition of "biasing potentials". Each biasing potential is used to enhance the sampling in a given region, typically referred to as a window. In umbrella sampling, one performs M simulations with different biasing potentials ,i=1...M, obtaining a probability distribution for each of the M windows.After compensating for the bias introduced by the potential , the M distributions are "stitched" together.