The interaction of slow atoms and ions with metal surfaces results in a hybridization of the atomic orbitals as well as a shift and a broadening of the associated energy levels. In perturbative treatments of the atom-surface interaction, hybridization effects can be taken into account approximately through a proper choice of the atomic wave functions. We examine the inclusion of hybridization effects in a first-order treatment by employing hydrogenic atomic wave functions in the parabolic (Stark) representation as well as in the spheroidal (two-center) representation. These two representations diagonalize different parts of the image-charge interaction. Level widths calculated from the parabolic and the spheroidal representation are compared to nonperturbative level widths. The parabolic representation appears to be a favorable choice for describing hybridization effects in first-order calculations.