For pure pair potentials the total energy can be broken down into a sum of binding energies for the individual atoms. This is coded for the Morse and LJ potentials. If the binding energy of any atom is lower than that of the most tightly bound atom multiplied by the variable astep (see the STEP command above) then that atom is randomly replaced on the sphere of radius equal to that of the atom furthest from the centre of mass. Hence if astep is zero angular steps will never be taken, and the closer this parameter is to one the more atoms will undergo such displacements. If the step sizes are adjusted to give the required acceptance ratio then both step and astep are multiplied or divided by every naccept steps depending upon the current acceptance ratio. If FIXSTEP has been specified then the temperature is adjusted in the same way and step and astep are unchanged.