Methylene: a paradigm for computational quantum chemistry.

The year 1970 has been suggested as a starting date for the "third age of quantum chemistry," in which theory takes on not only qualitative but also quantitative value. In fact, each of the years 1960, 1970, 1972, and 1977 is of historical value in the unraveling of the structure and energetics of the CH(2) molecule, methylene. What took place for methylene, namely the establishment of credibility for theory, has subsequently taken place for many other molecules. Three important roles for quantitative theory are outlined: (i) theory precedes experiment; (ii) theory overturns experiment, as resolved by later experiments; and (iii) theory and experiment work together to gain insight that is afforded independently to neither. Several examples from each of the three classes are given.