where each 2-body term is just a linear expansion using a basis of powers of Morse variables for the corresponding CC, CH, and HH internuclear distances. Here we used the same total polynomial order M for simplicity, but they can be different for CC, CH, and HH. The actual values used in this work are presented in Computational Details. The 3-b term, V 3-b is given explicitly by the sum of 4 distinct 3-b interactions, namely