Steve and others,
This is just a note that I would suggest putting into documentation. If pair_brownian is given a cutoff greater than the radius of the particle the log terms in the lubrication forces (a_sq, sh, and pu) become negative. This creates problems when trying to find the structure of the random forcing (which necessitates taking a sqrt), namely nan forces.
In general I would caution against using such large cutoffs for the force even in the pure lubrication case, e.g. in the high Pe number limit, as it switches the direction of the forcing.