Calculated microwave emission spectra of the P-1 (A), O-1 (B), O-2 (C) and O-3 (D) nanoclustersat T = 20 K normalized so that the most intense peak has unit intensity. For P-1 no clear separation between lines is found. For O-1, themost intense lines are separated by a line spacing of 1.85 GHz, which lowers to 1.3 Ghz for O-2. For O-3 the density of states becomes too large to identify a line-spacing, although some individual lines appear.