Dear Colleague,

Two bugs have been found in the Dissipative Particle Dynamics (DPD) code of 
DL_MESO 2.5 related to Lees-Edwards shearing in parallel and the Langevin 
barostat.

The parallel deport, export and import routines for Lees-Edwards shearing 
include determining possible destination processes for beads passing through the 
boundary or being in the boundary halo. For situations when the boundary does 
not move in a particular direction, however, a superfluous modification to the 
destination processes is made and particles are thus sent to the wrong 
processes. This can result in the system temperature settling to an artificially 
high value. Many thanks are due to Martin Walker (Durham University) for 
identifying this bug.

This bug can be fixed by making changes to the following subroutines in 
domain_module.f90: deport_shear, import_shear, importvariable_shear, 
importdensity_shear, export_shear, exportvelocity_shear and exportdensity_shear. 
In these subroutines, each instance of the following lines:

  IF (test1==0.0_dp) THEN
    id1 = id1 - 1
    test1 = test1 + side1
  END IF
  IF (test2==0.0_dp) THEN
    id2 = id2 - 1
    test2 = test2 + side2
  END IF

should be changed to:

  IF (test1==0.0_dp) test1 = side1
  IF (test2==0.0_dp) test2 = side2

The same two lines should also be inserted immediately after the line:

  test2 = MOD (dimz-shove2, sidez)

for CASE (4) in deport_shear.

The current implementation of the Langevin barostat for all available 
thermostats uses an incorrect value for the random parameter and a convergence 
condition for particle velocity iteration that becomes increasingly difficult to 
attain for very large numbers of particles. Many thanks are due to Yaser Afshar 
(Johannes Gutenberg University) for identifying these problems.

To fix these problems, line 13 in constants.f90 should be changed to

  REAL(KIND=dp), PARAMETER :: langepsilon=1.0e-6_dp

line 196 in config_module.f90 should be changed to

  sigmalang = SQRT (2.0_dp * bbaro * psmass * temp / tstep)

and for each Langevin barostat implementation in integrate_module.f90, i.e. each 
subroutine ending with _lang, the two separate expressions in the DO-WHILE 
loops:

  conv = conv + ABS (vxx (i) - nvx (i)) + ABS (vyy (i) - nvy (i)) + ABS (vzz (i) - nvz (i))

  converg = conv

should be respectively changed to

  conv = conv + (vxx (i) - nvx (i)) * (vxx (i) - nvx (i)) + (vyy (i) - nvy (i)) * (vyy (i) - nvy (i)) + &
               &(vzz (i) - nvz (i)) * (vzz (i) - nvz (i))

  converg = conv / REAL(3*(nsyst-nfsyst), KIND=dp)

The above changes to the Langevin barostat are also described in a revised 
version of the user manual, which can be downloaded from the DL_MESO website.

The revised version of the package with all of the above changes can be 
downloaded and decrypted using the same procedure as outlined in the original 
instructions emailed to registered users. Re-registration is not necessary if 
you are a current DL_MESO version 2.5 user.

Michael Seaton
2012-07-20