The dynamics of a DC SQUID is analogous to the classical dynamics of a
particle subject to conservative, damping, and driving forces in two
dimensions. The equations of motion define a trajectory on a potential
-energy surface derived from the conservative forces, the components o
f which correspond to different forms of stored energy in the SQUID, I
n the presence of a periodic driving force, half-integral Shapiro step
s are possible when the trajectory follows a zig-zag path between mini
ma of the potential surface. This description of the dynamics in terms
of a potential surface provides an intuitive, physical basis for prev
ious simulation results on half-integral Shapiro steps in a DC SQUID.