View article

The theory for the ab initio evaluation of potential energy gradients at the configuration interaction level of accuracy is presented, with special attention to the treatment of the various types of orbitals (frozen core, active, frozen virtual) which may arise. The new method has been used for a study of the D_3h and C_2v geometries of S₃. SCF and CISD calculations predict the D_3h structure to be lower than the C_2v by 9.3 and 2.3 kcal/mol, respectively, whereas CASSCF and MRCISD predict the C_2v structure to be the lowest by 8.9 and 8.2 kcal/mol, respectively, using good basis sets. These calculations support the prediction of Carlsen and Schaefer that both forms should be observable experimentally.