Dear Colleague,

A bug in the parallel version of the Dissipative Particle Dynamics code in 
DL_MESO 2.4 has been discovered.

For systems involving hard surfaces, no boundary halo data is required next to 
the surfaces: these boundaries are identified using the Boolean array srflgc for 
each subdomain. Therefore the export of particle data does not need to take 
place across boundaries containing hard surfaces. The export routines in 
domain_module.f90, however, depend upon each processor sending and receiving 
data to/from two different processors. The current implementation of omitting 
boundary halo data across hard surfaces would thus cause the program to hang due 
to orphaned MPI calls.

To solve this problem, the exportdata, exportvelocitydata and exportdensitydata 
subroutines in domain_module.f90 need to be modified so the relevant export 
routines are called regardless of the existence of a surface, but still ensure 
that no particle data is sent across solid boundaries. To achieve this, the 
position range for each direction needs to be modified if a solid boundary 
exists so that none of the particles can be added to the boundary halo message 
buffer. By inserting the following lines immediately before each CALL statement 
in the three subroutines:

      IF (srflgc(1)) final = -rhalo
      IF (srflgc(2)) begin = sidex + rhalo
      IF (srflgc(3)) final = -rhalo
      IF (srflgc(4)) begin = sidey + rhalo
      IF (srflgc(5)) final = -rhalo
      IF (srflgc(6)) begin = sidez + rhalo

the IF statements in the export routines will not be able to find a particle 
that qualifies for copying into a boundary halo.

The IF statements should also be removed from the lines calling the export 
routines. For instance, in exportdata, the following line (1554)

      IF (.NOT. srflgc(1)) CALL export (nlimit, 1, map (1), begin, final, sidex)

should be changed to

      IF (srflgc(1)) final = -rhalo
      CALL export (nlimit, 1, map (1), begin, final, sidex)

It should be noted that the serial version of the module, domain_module_ser.f90, 
does not need to be modified as it does not rely on synchronized MPI calls.

This bug fix has been applied to the current DL_MESO release and a corrected 
version may be downloaded by registered users of version 2.4 without re-
registering and decrypted using the same password.

Michael Seaton
2011-06-07