The dotted line connecting a with b represents an unspecified, but approximately constant interaction between a and b. For example, when a = vw and b = el, the a-b interaction is 0 by definition. Additionally for the systems studied here, the van der Waals interaction between the head and tail groups have a weak z-dependence, so we may use a dotted line when a = h and b = t. In these cases, we can, at our convenience, take the dotted line to be either connected (with a double line) or disconnected.