It doesn't happen with every atmosphere but it is directly linked to the complexity of the atmosphere. A non-volumetric atmosphere is very likely to work just fine, but a volumetric atmosphere may freeze the programm. With the parameters getting more complex and more volumetric functions being activated, the chance for a freeze-up rises accordingly. On the other hand, it sometimes works out just fine - I can't really put a finger on a certain factor that will guarantee a freeze up. While this is annoying, it is not a really bad problem. The one problem that really bugs me is the volumetric problem: Volumetrics suddenly turning into colour-interference patterns. This one really ruins my workday... Joerg