In molecular systems, positive and negative charges are separated, making them ideal systems to
examine electrostatic interactions. The attractive force between positive and negative charges is
balanced by repulsive ‘forces’ that are quantum-mechanical in origin. We introduce an ‘effective’
potential energy that captures the repulsion; it allows us to obtain fairly accurate estimates of
the bonding properties of molecular systems. We use units (e.g., kcal mol –1 for energy) that
emphasize the relevance of electrostatics to macroscopic behavior.