Hey all, This is a slightly long question and I appreciate your patience. I wanted to use a finite difference lookup table as my forcing, specifically 'ffx'. For this, I read in the finite difference array from a text file and used a piecewise linear interpolant routine to interpolate onto the SEM grid given by xm1(i,1,1,1) and ym1(i,1,1,1), where i=1,lx1*ly1*lz1*lelt. Now, this worked fine in my smaller test domain having 6 elements, calculated in just 1 processor. However, my actual problem size is big, having 252 elements on 48 processors and I am having trouble with parallelization. I tried to load the input finite difference forcing onto each mpi rank and then use the interpolation routine by using a previous suggestion by Paul. subroutine usrdat2 include 'SIZE' include 'TOTAL' parameter (my_array_size=10000) real my_array(my_array_size), work(my_array_size) n = my_array_size call rzero(my_array,n) call rzero(work. ,n) if (nid.eq.0) then open (unit=49,file='myfile') read(49,*) (my_array(k),k=1,n) close(49) endif call gop(my_array,work,'+ ',n) ! Sum over all processors return end However, I get a bunch of NaN s in the output log. I believe my troubles are related to the parallelization part. Would like to know how to proceed. Thanks, Saikat Saikat Mukherjee, PhD Student, Paul Research Group - http://www.me.vt.edu/mpaul/ Engineering Science and Mechanics, Virginia Tech.