In contrast to the weak interaction commonly found between low valence metals such as Ca and a polymeric material, considerable changes in photoelectron spectroscopy have recently been observed in experiment after evaporating Ca onto poly (9,9-dioctylfluorene) (PFO). Here, by performing calculations using Moller-Plesset perturbation theory, we show that the Ca/PFO interaction depends on the number of Ca atoms attaching to the PFO oligomer unit. A strong interaction could take place when two Ca atoms were adsorbed at a PFO unit. Further, the resulted PFO deformation and the modified frontier orbitals could be recovered when oxygen was further added. © 2005 Elsevier B.V. All rights reserved.