A new ab initio potential energy surface for the collisional excitation of HCN by para- and ortho-H2

Abstract

We present a new four-dimensional potential energy surface for the collisional excitation of HCN by H2. Ab initio calculations of the HCN-H2 van der Waals complex, considering both molecules as rigid rotors, were carried out at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12a] level of theory using an augmented correlation-consistent triple zeta (aVTZ) basis set. The equilibrium structure is linear HCN-H2 with the nitrogen pointing towards H2 at an intermolecular separation of 7.20 a0. The corresponding well depth is -195.20 cm(-1). A secondary minimum of -183.59 cm(-1) was found for a T-shape configuration with the H of HCN pointing to the center of mass of H2. We also determine the rovibrational energy levels of the HCN-para-H2 and HCN-ortho-H2 complexes. The calculated dissociation energies for the para and ortho complexes are 37.79 cm(-1) and 60.26 cm(-1), respectively. The calculated ro-vibrational transitions in the HCN-H2 complex are found to agree by more than 0.5% with the available experimental data, confirming the accuracy of the potential energy surface.

Rotational excitation of HCN by para- and ortho-H2

“…At first, it seems that the resonances are more pronounced for cross sections with para-H 2 ( j 2 = 0) than the collisions with H 2 ( j 2 ≥ 1). Nonetheless, Denis-Alpizar et al 30 reported that the total number of bound states for HCN-para-H 2 ( j 2 = 0) is 101 whereas for the HCN-ortho-H 2 ( j 2 = 1) 330 bound states were founded. There are around three times more states for the ortho form than for the para form of the complex.…”

“…42 The significant j 2 dependence of the cross sections is due to the large anisotropy of the PES with respect to H 2 rotations (see Ref. 30). …”

“…In our scattering calculations, we employed the recently computed HCN-H 2 PES of Denis-Alpizar et al 30 The accuracy of the new PES was estimated by comparing theoretical HCN-H 2 transition frequencies with the experimental ones of Ishiguro et al 31,32 The agreement between the theoretical and experimental results was found to be better than 0.5% showing that the PES can be used with confidence for the scattering calculations.…”

“…The use of explicitly correlated coupled clusters methods for generating van der Waals complex PES has been investigated in detail by Lique et al 33 and Ajili et al 34 The theoretical approach used in Ref. 30 is an extension to the case of two rigid rotors of the methodology used by Ajili et al 34 Then, it is expected that the present PES calculated at the CCSD(T)-F12a level together with diffuse basis set of aVTZ quality fully reaches the CCSD(T)/CBS accuracy. Both molecules were treated as rigid rotors fixing the internuclear distances of H 2 and HCN at its averaged and experimental equilibrium values, respectively: r HH = 1.448 a 0 for H 2 and r CN = 2.179 a 0 and r CH = 2.013 a 0 for HCN.…”

Potential energy surface of the CO2–N2 van der Waals complex

“…We can conclude that the CCSD(T)-F12a/aug-cc-pVTZ method performs very well for selected configurations at long-range separations which confirms again the validity of this methodology. 15,43,17 …”