Hertz-Mindlin Nassauer-Kuna (Polyhedral)
This model is a modification of the Hertz-Mindlin model where the effective radius is calculated using the Nassuaer-Kuna formula 
where V is the overlap volume and d is the penetration depth.
The effective radius, RNK is used to determine the equivalent radius, R* which is substituted into the Hertz-Mindlin formula to get:
All forces are multiplied by the correction factor of 0.62/0.752 specified in Based on: Nassauer, B., 2013.
This produces the following:
