Anomalies
Adjacent cluster carbons join two rings at their edges.
There are six of these in the common steroids, such as progesterone, resulting in a negative correction for this relatively flat alkane surface.
Bridged ‘cluster carbons’ create a cage-like carbon structure. It is tempting to predict that, with less surface per carbon in the ‘caged’ structure, they should require an even greater negative correction, but on the next slide we show that it is positive.