This work compares the performance of theoretical methods and basis sets on the molecular structure, atomisation and ionisation energies, electron affinity, and vibrational spectrum of silylene. Silylene, its cation and anion have been studied in 1 A 1 , 2 A 1 and 2 B 1 states, respectively, in the gas phase and C 2v symmetry. The methods considered are second-order Møller-Plesset perturbation theory (MP2), the density functional theory (DFT), Gaussian-2 (G2) and complete basis set methods (CBS-4M and CBS-Q). The basis sets used are 6-31G(d,p), 6-311G(d,p), 6-31++G(d,p) and 6-311++G(d,p). The functional used for the DFT method is B3LYP. Silylene and its cation and anion have been optimised using the MP2 and DFT methods and the named basis sets. Single-point energy calculations (G2, CBS-4M and CBS-Q) were performed using MP2/6-311++G(d,p) structures and these energies have been used to calculate atomisation energy, ionisation energy and adiabatic electron affinity. Frequency calculations were also done and the raw vibrational frequencies were assigned. It is interesting to note the close similarity between the predicted parameters and some of the available literature values. The results obtained are consistent and converge with different basis sets with improved size and quality. However, the parameters obtained are very much methoddependent.

The molecular structure, atomization energy, ionization energy, electron affinity and vibrational spectra of silicon dihalides, SiX 2 (X ¼ F, Cl, Br, I), have been obtained using theoretical methods in the gas phase. The neutral dihalides, their cations and anions have been studied in C 2v symmetry. The neutral species have been considered in both the 1 A 1 singlet and 3 B 1 triplet states whereas the cations (SiX 2 þ ) and anions (SiX 2 7) have been considered in the 2 A 1 and 2 B 1 states, respectively. The methods used are Møller-Plesset perturbation theory (MP2), density functional theory (DFT), Gaussian-2 (G2) and complete basis set methods (CBS-4 and CBS-Q). The functional used for the DFT method is B3LYP. The basis sets used are 6-311þþG(d,p) for all atoms except that 6-311G(d,p) has been used for iodine atom only. The silicon dihalides, their cations and anions have been fully optimized using MP2 and DFT methods. Single point energy computations (G2, CBS-4M and CBS-Q) have been performed except for silicon diiodide. The results obtained are in good agreement with literature although they are method dependent.