We show that the cross sections for molecule -molecule collisions in the presence of an external field can be computed efficiently using a total angular momentum basis, defined either in the bodyfixed frame or in the space-fixed coordinate system. This method allows for computations with much larger basis sets than previously possible. We present calculations for 15 NH -15 NH collisions in a magnetic field. Our results support the conclusion of the previous study that the evaporative cooling of rotationally ground 15 NH molecules in a magnetic trap has a prospect of success.