The pyridine dimer is considered in more detail in Fig. , where we plot energy component surfaces for twodimensional lateral displacements, using the same dipolealigned configuration of (pyridine) 2 that was considered ( monomer dipole moments. At R = 3.9 Å, these wells are clearly evident in E vdW (Fig. ), whereas E elst is strictly repulsive (Fig. ) and cannot explain offset stacking. At R = 3.4 Å, a double-well structure remains in E vdW but electrostatic interactions are attractive and serve to deepen the wells very slightly. A similar picture emerges if one pyridine monomer is rotated by 180 • with respect to the other, such that the monomer dipole moment vectors are antiparallel to one another. Two-dimensional contour plots for this case are provided in Fig. . The Hunter-Sanders model invokes E elst + E disp to explain offset stacking so this potential is considered in Fig. , plotted in a manner analogous to Fig. and using the same values of R. At both distances, the minimum on the E elst + E disp surface lies at the origin, meaning that the sum of electrostatics and dispersion prefers not to offset-stack. Figure shows that E elst + E exch does not recover offset-stacking in (pyridine) 2 either. (This combination of energy components mimics the antisymmetrized electrostatics used in some DFT-based energy decomposition schemes. ) Only when Pauli repulsion is included does offset-stacking arise, and it does not require electrostatics.