Quantum-chemical density functional theory (DFT) computations were applied to study the mechanism of surface desorption of methyl groups on the AlN surface. The calculations were performed using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional together with Grimme D3-empirical dispersion correction 17 using the Vienna Ab initio Simulation Package (VASP) . For the representation of pseudopotentials for Al, N, C and H the projected augmented wave (PAW) method provided by that package was used, where in the surface studies a fractional charge of 0.75 was used for the H atoms bonded to the N atoms at the bottom (opposite side) terminated surface layer to facilitate the saturation of dangling bonds. The hexagonal unit cell of the bulk AlN structure was first derived by energy minimization with respect to the lattice parameters and coordinates using a 777 grid of gamma-centred k-points, resulting in a = 3.114 Å and c = 4.995 Å. Previous experimental determination of the lattice parameters show that a = 3.11131 Å and c = 4.98079 Å which correlates well with the DFT calculations. Then a model of a 2-dimensional surface was constructed by cutting out a slab of 5 (Al, N) layers orthogonal to the c-direction (corresponding to a vertical height of about 10.6 Å from a "top" Al to a "bottom" N), with a hexagonal surface (2-dimensional) unit cell with asurf = 3.114 Å (and with the direction of the c-axis reversed compared to the crystal cell c-axis), see Fig. . The length of the c-axis was increased to 40.0 Å to create empty space between the slabs. A cell with the surface cell axes (asurf, bsurf where bsurf = asurf) doubled were used in the calculations, which led to 4 Al atoms (or "surface sites") with a dangling bond that can be saturated by CH3 being present at the topmost surface. The atomic coordinates of the slabs (with or without adsorbents) were geometry optimized using a 331 k-point grid. Free energies were calculated from the computed vibrational mode frequencies of the optimized structures (Table ).