What is the mean curvature of nitrates and nitrites?

I am reading the book *Soft Matter Physics - Synthetic and Biological Self-Assembling Materials* from Hamley and particularly page 201. 


I started to think how this packing occurs in case of molecules or *molecular patches*, particularly in case of nitrates and nitrites because they have highest priority set by WHO in their drinking water guideline. 
The curvature can be defined as the amount by which a geometric object deviates from being flat. 


You can assume that these molecules clamp together such that there is no other interactions. 


**How can you approximate the mean curvature of nitrates?**


