DFT and ab initio calculations are performed on tetramethyltin (SnMe4, TMSn) and its cation. A set of spectroscopic constants for both species are derived. They include equilibrium geometries, rotational constants and vibrational wavenumbers. All quantities are in close agreement with the available experimental data. For the cation, our calculations confirm the C3v charge transfer structure proposed earlier through the analysis of electron paramagnetic resonance (EPR) experimental data. Using multi reference configuration interaction and time dependant density functional theory (TD-DFT) methodologies, the vertical electronic excitation energies of TMSn and TMSn+ are determined. For the singlet-singlet neutral molecule, our calculated transition energies are distinctly lower than those previously computed. For the TMSn ($\tilde X$1A1 ? 11T2) absorption transition, our computed excitation energy coincides, however, with the experimental value. Predictive data are also given for the TMSn triplets. At the best level of theory, the vertical and the adiabatic ionization energies of TMSn are computed 9.86 eV and 8.74 eV, respectively. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012