The effects of basis set size and type on the theoretical optimum geometries calculated by ab initio SCF and CI methods is studied for the molecular species H2O 1A1, NH2 2B1, CH2 1A1, H2O+ 2B1, NH2 2A1, CH2 3B1, BH2 2A1, H2O+ 2A1, over the range from minimum to much extended basis sets. The difference in optimum geometry between SCF and CI calculations is also noted. Calculations made specially for this study have used double-zeta and double-zeta-plus-polarization Gaussian basis sets. The geometries predicted by these and many published calculations are plotted as points on diagrams of bond angle ϑ versus bond length r. It is observed that there are striking similarities between such diagrams for different species, and thus it is possible to use the diagrams in order to predict a molecular geometry with greater accuracy than by one method alone (except CI with a very extended basis). Such a prediction is made from calculations of the species NH−2 1A1.