Full-dimensional quantum dynamics calculations of rovibrationally inelastic scattering of CO-H$_2$

The CO-H$_2$ collisional system is crucial in determining the physics and chemistry of interstellar environments due to the astrophysical importance of H$_2$ and CO. We calculated for the first time a full-dimensional (6D) potential energy surface (PES) for this system using the high-level CCSD(T)-F12B method. The PES was fitted using an invariant polynomial method in 6D. Quantum close-coupling calculations of rotational and vibrational quenching of CO in collisions with H$_2$ were carried out on the new 6D PES. The pure state-to-state rotational excitations from CO($v_1=0$, $j_1$=0, 1) were benchmarked with crossed molecular beam measurement for collision energies of 795 - 991 cm$^{-1}$ and 3.3 - 22.5 cm$^{-1}$. The computed cross sections for $j_1=0 \rightarrow 1$ transition in CO and show better agreement with measurement than those obtained on a recently available 4D PES. For rovibrational transitions, state-to-state and total quenching cross sections and rate coefficients were calculated for the vibrational quenching in CO($v_1=1, j_1$)+H$_2$($v_2=0, j_2$) $\rightarrow$ CO($v_1^{\prime}=0, j_1^{\prime}$)+H$_2$($v_2^{\prime}=0, j_2^{\prime}$) collisions, $j_1=0, 2$ for para-H$_2$ and $j_1=1, 3$ for ortho-H$_2$. The results are compared with experimental results and previous calculations using 4D PESs and various decoupling approximations. Our calculation also confirmed that the contribution from a quasi-resonant channel, CO($v_1$=1) + H$_2$($v_2$=0, $j_2$=2) $\rightarrow$ CO($v_1^{\prime}$=0) + H$_2$($v_2^{\prime}=0$, $j_2^{\prime}$=6), dominates the vibrational quenching of CO in collision with para-H$_2$ for $T\geq 50$~K.