TDMP2 calculation of dynamic multipole polarizabilities and dispersion coefficients of the triplebonded molecules CO, N2, CN−, and NO+

Abstract

In order to test the performance of the recently developed time-dependent second-order Mo/ller–Plesset perturbation theory (TDMP2) for anisotropic frequency-dependent multipole polarizabilities, we have studied the isoelectronic series CO, N2, CN−, and NO+. The polarizabilities of these triple-bonded diatomics are an interesting test for the TDMP2 method, because it is known that Mo/ller–Plesset perturbation theory has its difficulties describing multiple bonds. We selected these molecules as test systems because, especially for N2 and CO, accurate experimental and other ab initio data are available to compare with and their dynamical polarizabilities are needed for dispersion coefficients of van der Waals complexes, which are presently under intensive investigation. To get reliable results near the TDMP2 basis set limit we used large one-particle basis sets, optimized for polarizability calculations at the coupled Hartree–Fock level. The results show that the TDMP2 method is capable to improve for the isotropic as well as for the anisotropic polarizabilities considerably on the TDHF approximation, with the exception of the dipole polarizabilities of N2 and NO+, for which the static correlation effects are too strong to be treated by second-order Mo/ller–Plesset perturbation theory. However, we find, that the TDMP2 method, due to the use of coupled (TDHF) first-order orbital rotation parameters, is somewhat more stable with respect to static correlation effects than the singles-doubles-triples many-body perturbation theory to second order [SDT-MBPT(2)] double perturbation theory. Where reference data are available, the TDMP2 results for static polarizabilities and for the first Cauchy moments are in good agreement with the best theoretical and experimental data. We also calculated dispersion coefficients for the (N2)2 and the (CO)2 dimer and, utilizing the results of previous TDMP2 studies for the atoms He through Xe, also for the respective rare gas complexes of N2, CO, CN−, and NO+. We estimate the results to be the most accurate ab initio data available for these van der Waals coefficients.