The traceless moments and polarizabilities of the CH 3 CN molecule are calculated using MP2 level of theory with the aug-cc-pVDZ basis , as the MP2 calculation of the dipole magnitude gives a value of 1.55 a.u., close to the experimental value of 1.54 1 . See section 2, and Tables in the Supporting Information for more details. These traceless moments and polarizabilities are used to compute the rotationally averaged moments and polarizabilities over the C 1v axis of the linear-MeCN. The basis vectors to calculate the rotational averaged moments and polarizabilities are shown in Figure . The z is chosen to align along the C-N bond vector and the two other basis vectors were constructed by the Gram-Schmidt orthogonalisation. For example, the expression for rotationally averaging the Cartesian quadrupoles