Determination of the headgroup-gold(111) potential surface for alkanethiol self-assembled monolayers by ab initio calculation

We present a realistic empirical potential function to model the head-group interaction for self-assembled monolayers (SAMs) of alkanethiols on Au(111). The potential function is fit to data obtained by ab initio geometry optimization of SCH3 on Au(111) clusters. The principal result from our calculations is that barriers within the surface corrugation potential are too small to pin S atoms at any particular site. We note that simulations of alkanethiol/gold systems that employ a model precluding lateral movement of S cannot reproduce all dynamical behavior of the SAM.