[modeller_usage] Negative molpdf values

Hi,

I am generating and refining models using the LoopModel or DOPELoopModel or DOPEHRLoopModel methods. Sometimes I get negative molpdf values for some of my generated models. What does this mean? I thought that the molpdf should be positive. The models with the most negative molpdf correspond to the most negative (i.e the lowest) DOPE scores - are these therefore the best models?

Thanks.