Analytic configuration interaction gradient studies of SH4, sulfurane

The potential energy surface of SH4, the prototype sulfurane, has been examined in light of previous theoretical studies suggesting that there may be two distinct energy minima. The first of these has the four H atoms in roughly octahedral positions, as in the experimentally known SF4 molecule, and belongs to point group C2v. The second isomer is square pyramidal, point group C4v. Molecular electronic structure theory has been applied using basis sets of triple zeta (TZ) and triple zeta plus polarization (TZ+P) quality. Configuration interaction (CI) including in some cases all single and double excitations was carried out and geometries optimized via newly developed analytic CI gradient techniques. At the TZ SCF and TZ CI levels of theory there are both C2v and C4v relative minima, separated by +2.4 and −1.6 kcal, respectively. However, using the more complete TZ+P SCF and TZ+ P CI methods, the C2v minimum disappears, leaving only the C4v minimum. Geometries corresponding to the expected C2v minimum lie ...