Abstract: The interaction potential of a He--BH complex is investigated by the coupled-cluster single-double plus perturbative triples (CCSD (T)) method and an augmented correlation consistent polarized valence (aug-cc-pV)5Z basis set extended with a set of (3s3p2d1f1g) midbond functions. Using the five two-dimensional model potentials, the first three-dimensional interaction potential energy surface is constructed by interpolating along ($r-r_{\rm e}$) by using a fourth-order polynomial. The cross sections for the rovibrational relaxation of BH in cold and ultracold collisions with ³He atom are calculated based on the three-dimensional potential. The results show that the $\Delta v = - 1$ transition is more efficient than the $\varDelta v = - 2$ transition, and that the process of relaxation takes place mainly between rotational energy levels with the same vibration state and the $\varDelta j = - 1$ transition is the most efficient. The zero temperature quenching rate coefficient is finite as predicted by Wigner's law. The resonance is found to take place around 0.1--1~cm^{ - 1} translational energy, which gives rise to a step in the rate coefficients for temperatures around 0.1--1~K. The final rotational distributions in the state $v = 0$ resulting from the quenching of state ($v = 1$, $j = 0$) at three energies corresponding to the three different regimes are also given.

Key words: He--BH complex, potential energy surface, cold collision, cross section