The ground state of the cations Li+ through Kr+ has been studied by using explicitly correlated wave functions. The variational wave function has been previously used [E. Buendia, F.J. Galvez, A. Sarsa, Chem. Phys. Lett. 428 (2006) 241] to study the corresponding neutral atoms. Near degeneracy has been also considered by means of a multi-configurational model wave function for a number of neutral atoms and cations. Starting from the total energy of the cations here obtained along with previously reported neutral atom energies (except for some cases here discussed), the ionization potential of these atoms has been calculated by including relativistic effects perturbatively. The calculation has been carried out by using the variational Monte Carlo method. (c) 2007 Elsevier B.V. All rights reserved.