Walsh applied his approach to molecules other than compounds of hydrogen. but the correlation diagrams soon become very complicated. His approach represents a valuable complement to the VSEPR model because it traces the influences on molecular shapes of the occupation of orbitals spreading over the entire molecule and concentrates less on localized repulsions between pairs of electrons. Correlation diagrams like those introduced it by Walsh are frequently encountered in contemporary discussions of the shapes of complex molecules, and we shall see a number of examples in later chapters. They illustrate how inorganic chemists can sometimes identify and weigh competing influences a by considering two extreme cases (such as linear and 90° XH2 molecules), and then rationalize the fact that the state of a molecule is a compromise intermediate between the two extremes.