Restricted-spin coupled-cluster single and double plus perturbative triple excitations [RCCSD (T)] and/or complete-active-space self-consistent-field multireference internally-contracted configuration interaction (CASSCF/MRCI) potential energy functions of the (X) over tilde B-2(1), (A) over tilde B-2(2), (B) over tilde (2)A(1), and (C) over tilde (2)A(2) states of Cl2O+ and the (X) over tilde (1)A(1) state of Cl2O, with basis sets of up to the augmented-correlation-consistent-polarized-valence-quadruple-zeta quality, have been reported. For each of these states, vibrational wave functions of the symmetric stretching and bending modes have been computed, employing the potential energy function obtained at the highest level of calculation, with Watson's Hamiltonian and anharmonic vibrational wave functions expressed as linear combinations of harmonic basis functions. The helium I photoelectron spectrum of Cl2O has been simulated with Franck-Condon factors calculated using computed anharmonic vibrational wave functions and allowing for Duschinsky rotation. The adiabatic ionization energies (AIEs) to the four lowest cationic states of Cl2O+ have been evaluated at the RCCSD(T) level with basis sets of up to polarized-valence-quintuple-zeta quality and by various extrapolation techniques to the basis set limit. Revised equilibrium geometrical parameters of the (X) over tilde B-2(1) and (C) over tilde (2)A(2) states of Cl2O+ were obtained from the iterative Franck-Condon analysis procedure, and revised AIEs for the (A) over tilde B-2(2) and (B) over tilde (2)A(1) states of Cl2O+ were estimated based on comparison between the simulated and observed photoelectron spectra. It was found that inclusion of anharmonicity in the Franck-Condon factor calculations for each electronic state improves the quality of the simulated spectrum. The computed T-1 diagnostics from the RCCSD calculations suggest that the (B) over tilde (2)A(1) state of Cl2O+, with the ...(9a(1))(1)(3b(2))(2)(10a(1))(0) electronic configuration, possesses multiconfigurational character in the region of r(ClO)=1.87 Angstrom and theta(ClOCl) = 125degrees. CASSCF/MRCI/aug-cc-pVQZ(no g) calculations show an avoided crossing between the (B) over tilde (2)A(1) state and (2)(2)A(1) state [with the ...(9a(1))(2)(3b(1))(0)(10a(1))(1) electronic configuration], in the region of 1.96>r>1.80 Angstrom and 137.0>theta>132.0degrees. (C) 2003 American Institute of Physics.