A Coupled Cluster Study of van der Waals Interactions of the He Atom with CN, NO and O2 Radicals

The partially spin-adapted coupled cluster method with the restricted open-shell Hartree- Fock reference was applied to calculations of interaction energies between the helium atom and the three radicals, CN (2Σ), NO (2Π), and O2 (3Sg-). Basis set dependences with medium-augmented correlation consistent basis sets were alleviated by using extrapolations to the basis set limit which were based on aug-cc-pVTZ and aug-cc-pVQZ results. The two-dimensional potential energy surfaces were fitted by exponential and polynomial functions. Minima and transition states were located. Potential energy surfaces are very floppy, especially for HeCN. This complex exhibits the weakest van der Waals interaction, the electronic interaction energy being 92 μEh. Interaction energy in HeNO is 122 μEh, almost the same as was found for HeO2 (124 μEh). Considering zero-point-vibrational corrections, the dissociation energy of HeCN, HeNO, and HeO2 is 4.6, 6.6, and 7.3 cm-1, respectively. This sequence of the magnitude of interaction energies and the structural data for global and local minima and transition states were compared with available literature data. No simple link between the magnitude of intermolecular forces and dipole moments and dipole polarizabilities of CN, NO, and O2 was found. The low-order long-range model based on the induction and dispersion forces is completely useless in the assessment of the sequence of the size of intermolecular interactions of the HeCN, HeNO, and HeO2 complexes.