R. Ab-initio, CH 3 keeping C 3v symmetry and fixed C-H distance: only umbrella angle ? He position relative to CH 3 center of mass in spherical coordinates R,?,? Several 3D surfaces assembled into 4D surface including CH 3 relaxation Overall about 3000 potential energy points Analytical representation with angle dependent HFD form expanded over real spherical harmonics T lm with symmetry restrictions on l,m: V(R,?,?) = A exp{-b(?,?), p.1