FitRPBMFunction[tms,dat]

fits the RPBM function to the data dat using the mixing times tms using FindMinimum. The output is a rule of the fitted parameters {d0->val, tau-> tau, zeta ->zeta}.

FitRPBMFunction[tms,dat,init]

does the same but uses init as initial guess values.

FitRPBMFunction[tms,dat,init,fix]

does the same but uses init as initial guess values and fixes the parameters in fix.