Performance of theoretical methods and basis sets on the molecular structure, atomisation and ionisation energies, electron affinity, and vibrational spectrum of silylene

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 method dependent.