Mean force on the terminal basepairs need to maintain them at a given separation, as a function of that separation distance. Shown are atomistic simulations for the fully paired first basepair (red), first basepair 10% frayed and 90% (orange dashed line), and 30% frayed and 70% paired (light-brown dashed line), together with the results of Cocco et al. (37) (green dashed line), the PB model (dark-blue dotted line), and coarse-grained simulations (pink dotted line). We also show results (in light blue and black) for coarse-grained simulations of the same sequence with the first basepair and second basepairs changed from C-G to A-T. The vertical lines mark the distances beyond which the first and second basepairs are fully unzipped. To see this figure in color, go online.