Non-bonded force computation get very inefficient for inhomogenous systems

The NBNxM module sets the size of the cell in x/y for the (cluster-)pair search grid based on the average density in the system. If the system is very inhomogeneous, e.g. contains a lot of vacuum, these cells will be too large. This causes cluster pairs to become very flat which leads to many more cluster pairs ending up in the pair list. The can decrease the performance by one or more orders of magnitude for a system or e.g. a droplet in a large box with vacuum/vapor.