Accurate equilibrium properties of the ground and the three lowest excited states of the trimethylenemethane (TMM) diradical are calculated by using the novel spin-flip electronic structure method. Changes in structures and vibrational frequencies upon excitation are analyzed. The bonding picture in different states of TMM is derived from wave function analysis and comparison of equilibrium structures with typical values of coupled-cluster (CC) bond lengths, e.g., a double CC bond in ethylene, a single CC bond in twisted ethylene, and a bond in the allyl radical. (C) 2003 American Institute of Physics.