Ab initio study of He2H+ and Ne2H+:: Accurate structure and energetics

An ab initio investigation employing highly correlated electronic methods and large basis sets has been carried out to determine the equilibrium geometries and binding energies of He2H+ and Ne2H+. The theoretical methods employed include the Moller-Plesset Perturbation Theory (MP2, MP4), and coupled cluster with single and double substitution with noniterative triple excitation [CCSD(T)] ab initio methods with two different type of basis sets, the segmented contracted 6-311 + +G basis sets with multiple polarization functions, and the correlation consistent aug-cc-pVxZ (x = D, T, Q, 5) basis sets. It is found that both species have in common linear symmetric XHX+ equilibrium geometries with dissociation energies of more than 4000 cm(-1) to X + XH+ (X = He, Ne). A convergence study comparing the uncorrected and counterpoise (CP) corrected dissociation energies with respect to the complete basis set (CBS) limiting values shows that the CP method generally yields less accurate dissociation energies than the uncorrected ones in both ionic species; a possible explanation is given in terms of differences in geometries between the dissociated and complex state. The dissociation energies for He2H+ to He+HeH+ at MP4/aug- cc-pV5Z and CCSD(T)/ aug-cc-pV5Z levels are, in cm(-1), 4622.2(4621.7) and 4631.1(4631.7), respectively, with the values in parentheses representing the dissociation energies at the CBS limit. The corresponding dissociation energies for Ne2H+ at MP4/aug-cc-pVQZ and CCSD(T)/ aug-cc-pVQZ levels are 5846.9(5746.3) and 5807.1(5703.9), though the estimated CBS limit in this case is less reliable than in the case of He2H+. (C) 1999 American Institute of Physics. [S0021-9606(99)30509- 2].