State to state rotational integral cross sections and rate coefficients of HCP collision with He at low temperature

We report coupled-cluster [CCSD(T)] ab initio calculations of the two-dimensional interaction potential energy surface of the HCP-He complex. The aug-cc-pVTZ and aug-cc-pVQZ gaussian basis sets are used. HCP is held fixed at its linear equilibrium ground vibrational level with the corresponding [H-C] and [C-P] bond lengths set to the values 2.016 bohrs and 2.914 bohrs, respectively. Our calculations are corrected for basis set superposition errors (BSSE). The PES obtained with the above triple zeta basis set has two minima located 22.018 cm−1 and 14.808 cm−1 below the HCP+He dissociation limit. These well depths are shifted to 23.505 cm−1 and 15.949 cm−1, respectively, when the quadruple zeta basis set is used. Our PESs are fitted on a basis of Legendre polynomial functions and state to state rotational integral cross sections of the HCP collision with He are calculated in the close-coupling (CC) approximation. Downward rate coefficients are inferred at low temperature (T⩽200K) by averaging the cross sections over a Maxwell-Boltzmann velocity distribution. An analysis of our results shows that for kinetic energies greater than ∼60 cm−1, the 0→2 transition dominates. The numbers derived here may be very useful for astrophysical observations.