Two types of structural problems are distinguished in relation to the nature of the quantumchemical methods suitable for their resolution. One problem concerns the chemical conclusions that can be drawn from intercomparison of the structures of a family of related molecules. For such a study, relatively low-level computational methods can be employed, because residual errors for a given structural parameter due to neglect of electron correlation and use of only a mediumsized Hartree-Fock basis set are nearly invariant to minor changes in the rest of the molecule. Such a study is illustrated by a set of structural evaluations of oxirane (ethylene oxide) and of all its possible fluorinated derivatives. The other class of problems consists of situations in which absolute values of structural parameters are required, either because the molecule under study has no close chemical relatives or because the chemically interesting question requires knowledge of the absolute value of one parameter for its resolution. Such a problem is illustrated by a study of the barrier inhibiting torsion about the CC bond of tolane (diphenylacetylene). The structural effects of successive fluorination of oxirane are found to parallel very closely those previously reported for fluorinated cyclopropanes. Even minor effects, such as the greater shortening of a C-F bond when it is cis to another C-F group compared to the shortening when it is in a trans configuration, are reproduced. The large-scale calculations on tolane, C 6K 5-CC-C 6H 5, made using a 6-311G ∗∗ basis set and MP2 treatment of electron correlation would have exceeded the storage capacity of our Cray computer without use of a new “direct correlation” method which makes unnecessary the storage of two-electron integrals over atomic basis functions. The equilibrium geometry of tolane is found to have the two phenyl rings coplanar. A barrier of 0.64 kcal mol −1 is found at the energy maximum at a torsional angle of 90 °.