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Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile,CH$_3$CN) and methyl isocyanide (CH$_3$NC) of interest here, have been observedin various astrophysical environments. The accurate modeling of their abundancerequires the calculation of rate coefficients for their collisional excitationwith species such as He atoms or H$_2$ molecules at low temperatures. In thiswork we compute new three-dimensional potential energy surfaces for theCH$_3$NC-He and CH$_3$CN-He van der Waals complexes by means of the explicitlycorrelated coupled cluster approach with single, double and perturbative tripleexcitation CCSD(T)/F12a in conjunction with the aug-cc-pVTZ basis set. We finda global minimum with $D_e= 55.10$ and 58.61 cm$^{-1}$ for CH$_3$CN-He andCH$_3$NC-He, respectively, while the dissociation energy $D_0$ of the complexesare 18.64 and 18.65 cm$^{-1}$, respectively. Low energy scattering calculationsof pure rotational (de-)excitation of CH$_3$CN and CH$_3$NC by collision withHe atoms are carried out with the close-coupling method and the collisionalcross sections of $ortho-$ and $para-$CH$_3$NC and CH$_3$CN are computed forkinetic energies up to 100 cm$^{-1}$. While the PESs for both complexes arequalitatively similar, that of CH$_3$NC-He is more anisotropic, leading todifferent propensity rules for rotational excitation. For CH$_3$NC-He, we findthat |$\Delta j$| = 1 transitions are dominant at low kinetic energy and apropensity rule that favors odd $\Delta j$ transitions is observed, whereas forCH$_3$CN the dominant cross sections are associated to transitions with|$\Delta j$| = 2. We expect that the findings of this study will be beneficialfor astrophysical investigations as well as laboratory experiments.

Interaction of CH$_3$CN and CH$_3$NC with He : potential energy surfaces and low-energy scattering