LB particle coupling bug

[Jean-Noël Grad]

Hi,

A severe bug was discovered in the LB particle coupling of ESPResSo [1]. 
In the unlikely event where a particle would find itself outside the 
simulation box with PBC by a tiny amount, for example at position 
(-1e-30, -1e-30, -1e-30), it could skip the LB particle coupling code 
due to rounding errors. This bug affects the CPU implementation of the 
LB method in the python branch and in releases 4.2.1, 4.2.0, 4.1.4, and 
most likely earlier releases too.

The probability of this bug occurring is low, but can be enhanced in 
simulations that purposefully place a lot of particles close to a 
periodic boundary: polymers sheared by Lees-Edwards boundary conditions, 
raspberry particles (colloids, bacteria, etc.) when crossing a periodic 
boundary, or cell membranes placed close to a periodic boundary.

A bugfix was pushed to the python branch today and to the 4.2.2 release 
candidate [2]. A list of all bugfixes in 4.2.2 can be found in [3].

Best regards,
JN

Links:
[1] https://github.com/espressomd/espresso/issues/4825
[2] https://github.com/espressomd/espresso/pull/4751
[3] https://github.com/espressomd/espresso/wiki/es-422-release-notes