Density functional calculations of neutral and anionic niobium trimer monoxides are presented. The calculations were performed employing scalar quasirelativistic effective core potentials. In order to test the accuracy of the used effective core potentials in the framework of density functional theory the pulsed field ionization-zero electron kinetic energy photoelectron spectrum of Nb3O was simulated and compared to experiment. Different isomers of Nb3O and Nb3O- were studied in order to determine the ground state structures. For both neutral and anionic systems a planar C-2v structure with an edge-bound oxygen atom was found as a ground state. Equilibrium structure parameters, harmonic frequencies, and adiabatic electron affinity are reported. The calculated electron affinity and frequencies are in good agreement with the available experimental data obtained recently from vibrationally resolved negative ion photoelectron spectroscopy. (C) 2004 American Institute of Physics.