Rotationally inelastic collisions of methinoposphide (HCP) with para-H(2) at low temperature.

The interaction potential energy surface of the methinoposphide (HCP)-H(2) complex is calculated at the ab initio coupled-cluster level of theory with an aug-cc-pVTZ Gaussian basis set. The [H-C] and [C-P] bond lengths of HCP are set to their values at the linear equilibrium ground vibrational level of the molecule. The calculated interaction energy presents two minima located 106.3 and 67.6 cm(-1) below the HCP+H(2) dissociation limit. Using the interaction potential obtained, we have computed collision excitation cross sections in the close-coupling approach and downward rate coefficients at low temperature, i.e., T<or=70 K. These quantities are significantly magnified in comparison with their counterparts for the HCP-He collisions. It is shown that there is a propensity toward DeltaJ=1 transitions.