Theoretical studies were carried out on the ground state of Si3. Two low-lying states were identified, namely, a singlet state of
symmetry with a bond angle of around 80°, and a triplet state of
symmetry. The two spin states are predicted to be very close in energy at their optimized geometries, with, for example, the ab initio methods with CCSD(T) and CASSCF(12,12) predicting the singlet state to be lower in energy by between 0.06 and 4.6 kcal/mol, respectively; whereas the DFT methods at (U)B3LYP/cc-pVQZ level predicting the opposite order with an energy difference of 1.62 kcal/mol. Walsh diagram is presented to explain the preferred spin state at the two different bond angles; and the bonding structures for the two isomers are proposed in light of the natural bond order analysis results by using the resonance hybrids model. However, unlike the energies, the two spin states are predicted to have fairly different dipole polarizability values: the mean
for the singlet and 100.4 for the triplet (in atomic units) at the (U)CCSD/Sadlej level of theory.
