  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 body-fixed 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 15NH - 15NH
collisions in a magnetic field. Our results support the conclusion of the
previous study that the evaporative cooling of rotationally ground 15NH
molecules in a magnetic trap has a prospect of success.
