The adsorption of N-2 gas on the LiF(0 0 1) surface is studied by canonical Monte Carlo (CMC) computer simulation. These results show that N-2 forms an ordered structure where the molecules are arranged in a unit cell of p(2 root 2 x root 2) R45 degrees symmetry at temperatures below 23 K with 50% coverage. The nitrogen molecules are tilted by 53 degrees from the surface normal and have the same azimuthal orientation along diagonals, with diagonals alternating their orientation. Beyond 23 K, the molecules become azimuthally disordered but with residual short-range order. No change in the position of the peak of the polar (tilt) angle distribution was observed above the transition temperature. This transition is purely of the order-disorder type. (C) 2009 Elsevier B. V. All rights reserved.