Dear Colleague,

A bug has been found in the Dissipative Particle Dynamics (DPD) code of DL_MESO 
2.5, and a minor change to improve usability is suggested.

Incorrect expressions are currently given for the calculation of potential 
energy and forces for the hard-core Lennard-Jones and Weeks-Chandler-Andersen 
potentials. This is due to incorrect values of the potential array being 
selected in the force calculation routines in field_module.f90. (No errors are 
made for DPD and many-body DPD interactions.) Many thanks are due to Chengwei 
Zhang for bringing this bug to our attention.

To fix the above bug, changes are needed in both config_module.f90 and 
field_module.f90. In config_module.f90, comment out lines 1678 and 1828, replace 
line 1683 with

  vvv (5, k) = aa

and replace line 1833 with

  vvv (5, k) = 0.25_dp * aa

In field_module.f90, replace lines 140, 151, 407, 418, 853, 864, 1133, 1144, 
1422 and 1433 with

  vk1 = (vvv (3, k) / rsq)**3

replace lines 146, 413, 859, 1139 and 1428 with

  IF (rsq<=vvv (4, k)) THEN

replace lines 152, 419, 865, 1145 and 1434 with

  vk2 = vvv (5, k)

and replace lines 154, 421, 867, 1147 and 1436 with

  pe = pe + vk0 * vk1 * (vk1 - 1.0_dp) + vk2

To make restarting DPD simulations involving frozen bead walls easier, the 
following change is suggested to remove the need for changes to be made to the 
FIELD file. In config_module.f90, change lines 212 to 216 to:

  IF (kres>0 .AND. nfsyst>0) THEN
    srftype = 0
  ELSE
    CALL frozenbead
  END IF

The above change will ensure the correct number of frozen beads is added to 
system totals when restarting simulations, unless a non-zero number of frozen 
beads is specified in the FIELD file.

We strongly recommend that all registered users of DL_MESO 2.5 should either 
make the code changes listed above or re-download the package using the 
instructions received on registration to get the corrected version of the 
software. Re-registration is not required (unless you have not yet upgraded to 
version 2.5) and the same password is used for decryption.

Michael Seaton
2012-08-22