DihedralTraPPE

class DihedralTraPPE : public feasst::BondFourBody

See the Dihedral section of http://trappe.oit.umn.edu/

$U(\varphi )=c_0+c_1[1+\cos (\varphi )]+c_2[1-\cos (2\varphi )]+c_3[1+\cos (3\varphi )]$