Dear Colleague,

A couple of bugs have been discovered in the Dissipative Particle Dynamics (DPD) 
code of DL_MESO 2.5.

The virial terms from bond stretching interactions have been found to be 
incorrectly calculated. While the forces and potential energies are generally 
calculated correctly, the resulting system pressures are found to be incorrect 
(normally too high): this is a particular problem for constant-pressure 
simulations using a barostat. Many thanks are due to Ming-Tsung Lee at Rutgers 
University, New Jersey for bringing our attention to this problem and helping to 
work out a fix.

To fix the above problem, change lines 618-627 and lines 1014-1023 in 
bond_module.f90 to:

  vir = vir + bforce * rsq
  ivrl (1) = ivrl (1) + bforce * xdif * xdif
  ivrl (2) = ivrl (2) + bforce * ydif * ydif
  ivrl (3) = ivrl (3) + bforce * zdif * zdif
  strsxx = strsxx - bforce * xdif * xdif
  strsxy = strsxy - bforce * xdif * ydif
  strsxz = strsxz - bforce * xdif * zdif
  strsyy = strsyy - bforce * ydif * ydif
  strsyz = strsyz - bforce * ydif * zdif
  strszz = strszz - bforce * zdif * zdif

The force term for Morse bonds was also found to be incorrect. This problem can 
be fixed by changing line 403 in bond_module.f90 to:

  force = 2.0_dp * welldepth * wellsize * term * (1.0_dp - term) / r

The expression for the Marko-Siggia bond potential was correct in the DPD code 
but incorrectly stated in the user manual. A corrected version of the user 
manual can now be downloaded from the DL_MESO website (www.ccp5.ac.uk/DL_MESO).

We strongly recommend that all registered users should either make the code 
changes listed above or re-download DL_MESO using the instructions they received 
on registration to obtain the corrected version of the software. Re-registration 
is not required and the same password is used for decryption.

Michael Seaton
2014-06-14