The molecule SiNN was investigated with the highly correlated coupled cluster method with single and double excitations and corrections for the connected triples [CCSD(T)], and with the multireference single and double excitations configuration interaction approach (MRCI). A series of basis sets ranging from the simple 6-31G*, 6-311G*, and Dunning’s double-zeta (DZ) plus polarization basis sets to the mole extended correlated consistent cc-pVTZ and cc-pVQZ ones were employed to check the dependence of the geometry optimization and of the frequency evaluation on the basis sets. Our highest level result of 1859 cm(-1) for the harmonic stretching frequency of the NN bond differs considerably from a previous CCSD(T)/DZ prediction of 1726 cm(-1). In fact, most of the results analyzed in this study point to a frequency value greater than 1810 cm(-1). Reexamining the scant experimental evidence, we estimate the harmonic frequency to be very close to 1830 cm(-1). Surprisingly, an extended internally contracted MRCI calculation with the cc-pVTZ basis predicts a frequency 144 cm(-1) higher than the corresponding CCSD(T)/cc-pVTZ result. Comparisons with existing density functional studies and with a previous MRCI calculation are also carried out. At the CCSD(T)/cc-pVTZ level, other isomers of SiN2, are further investigated for the first time. One symmetric linear ((3) Sigma(g)(-)), and one symmetric bent ((1)A(1)) structure are found to lie relatively high in energy : 84.60 and 102.23 kcal/mol, respectively, relative to SiNN ((3) Sigma(-)). However, one cyclic (1)A(1) and the singlet asymmetric isomer SiNN ((1) Sigma(+)) are only 6.09 and 15.81 kcal/mol above the global minimum. Although the higher frequencies of the former isomers do not fall in the region of relevance to the experimental assignment discussed in this work, that of the lowest lying (1)A(1) state (1850 cm(-1)) is practically identical to the CCSD(T)/cc-pVTZ frequency computed for the (3) Sigma(-) state