The first term, E elst , is the electrostatic interaction between the molecules due to intrinsic moments of the isolated molecules as well as due to interaction between the molecule and the electric field originating from the other molecules (or other external perturbation) while the second term, E non elst , is the non-electrostatic term that accounts for repulsive exchange and attractive dispersion interactions. The description of the electrostatic interaction is discussed in section 2.1. It is analogous to what has been presented for H 2 O. Then, the non-electrostatic contributions to the interaction energy are discussed in section 2. The electrostatic interaction site is placed on the center of mass of the molecule (red diamond), and the three repulsion and dispersion interaction sites are placed at the atomic coordinates of the C and N atoms, while the placement of the methyl group (Me) site is treated as a parameter (yellow circles) (see section 2.2.1). The C-N bond vector defines the z-axis whereas the two other axes, x and ỹ, are chosen orthogonal to the z-axis by using the Gram-Schmidt orthogonalisation procedure. The local-to-global transformation matrix consists of the {e X , e Y , e Z } unit vectors along the orthogonal axes set. The circular rotation angle ✓ indicates the arbitrariness of the orientation of the x-and ỹ-axes set, and is used as an integration variable in order to rotationally average the electrostatic properties about the rotationally invariant z-axis (see section 2.1.1).