The biochemical and physiological importance of nitric oxide (NO) in signaling and vasodilation has been studied for several decades. The discovery of both protein-bound and free low molecular weight dinitrosyl iron complexes (DNICs) suggests that such compounds might play roles in biological NO storage and transport. These complexes have important distinguishing spectroscopic features, including EPR and Mossbauer spectra, and NO vibrational frequencies (nu((NO))). The latter are particularly sensitive to modifications of the ligand environment and metal oxidation states. Examinations of functionals and basis sets delineate their effect on the NO vibrational frequencies and allow development of a methodology to calculate these frequencies in other DNICs. Three complexes of the form (L)(CO)Fe(NO)(2) (L = CO, N,N'-dimethyl-imidazol-2-ylidene (IMe) or 1-methylimidazole (MeImid)), where {Fe(NO)(2)}(10) is in its reduced form, have been used to calibrate the vibrational frequencies. The functional BP86 paired with a basis set of SDD/ECP on the metal and 6-311++G(d,p) on the ligand atoms exhibits the most accurate results, with deviations from experimental vibrational frequencies of no more than +/- 40 cm(-1). Subsequent investigations were performed on a series of diiron trinitrosyl complexes of the form {Fe(NO)}(7)-{Fe(NO)(2)}(9) bridged by sulfurs, namely, [(ON)Fe(mu-S,S-C6H4)(2)Fe(NO)(2)](-), [Fe(NO)(2){Fe(NS3)(NO)}-mu-S,S'], and [(ON)Fe(bme-dach)Fe(NO)(2)-mu-S,S'](+), with the ideal functional/basis set pair determined via the aforementioned test set. The ground state energetics (singlet/triplet/singlet, respectively), geometric parameters, and nitrosyl vibrational frequencies were calculated. The results for the former two complexes correlated well with the experimental work, and in contrast with what was reported in an earlier computational study, a stable triplet ground state structure was optimized for [Fe(NO)(2){Fe(NS3)(NO)}-mu-S,S']. For [(ON)Fe(bme-dach)Fe(NO)(2)-mu-S,S'](+), whose synthesis and structure were recently reported, the geometric parameters, vibrational frequencies, and total energies compare well to experimental ones and favor a singlet ground state.