The H2O2+. radical cation has been investigated with ab initio MO methods using both the single reference and multiconfiguration based wave functions. For the electronic states of two low lying isomers of the trans-(B-2(g)) and cis-((2)A(2)) forms, the effect of the choice of the reference wave function on the shape of the potential energy surfaces near the equilibrium geometry and also at the dissociation limit (OH. and OH+) was studied. In addition to the standard ab initio methods such as MP(n), CISD, CCSD(T), BD(T), and CASSCF, two different density functional methods were also employed. The force constants for the asymmetrical vibrations evaluated with the MP2 method are very large and consequently the corresponding nu(5) (b(u), or b(2)) calculated frequencies approach unphysical numbers, which are orders of magnitude larger than the expected. In addition the nu(2) (b(u)) frequency, which corresponds to the trans-conformer in the proper range around 1350 possesses a huge IR intensity. For the cis-structure even the first asymmetrical frequency nu(4) (b(2)), is too high (2216-2444 cm(-1)), and has an unreasonable IR intensity. It is shown that the symmetry breaking of the wave function is fully responsible for this unphysical behavior which leads to unreliable predictions of the whole vibrational spectrum. Detailed analyses of the symmetry break in in the calculations of harmonic vibrational frequencies are presented. Finally, reliable, vibrational spectra for both the H2O2+. isomers based on the full valence CASSCF and BD(T) methods are presented.