Geometries and energy separations of the various low-lying electronic states of Nb-n and Nb-n(-) (n=4,5) clusters with various structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field method followed by multireference singles and doubles configuration interaction (MRSDCI) calculations that included up to 52x10(6) configuration spin functions have been used to compute several electronic states of these clusters. The ground states of both Nb-4 ((1)A', pyramidal) and Nb-4(-) (B-2(3g), rhombus) are low-spin states at the MRSDCI level. The ground state of Nb-5 cluster is a doublet with a distorted trigonal bipyramid (DTB) structure. The anionic cluster of Nb-5 has two competitive ground states with singlet and triplet multiplicities (DTB). The low-lying electronic states of these clusters have been found to be distorted due to Jahn-Teller effect. On the basis of the energy separations of our computed electronic states of Nb-4 and Nb-5, we have assigned the observed photoelectron spectrum of Nb-n(-) (n=4,5) clusters. We have also compared our MRSDC1 results with density functional calculations. The electron affinity, ionization potential, dissociation and atomization energies of Nb-4 and Nb-5 have been calculated and the results have been found to be in excellent agreement with the experiment. (C) 2004 American Institute of Physics.