function moveatom(x1,y1,x2,y2)

% move an atom from location (x1,y1) to (x2,y2) in nmove steps,
% plotting along the way
%

nmove = 4;

% intermediate locations to plot motion:
dx = (x2-x1)/nmove;
dy = (y2-y1)/nmove;
x = x1 + (0:nmove)*dx;
y = y1 + (0:nmove)*dy;

for n=1:nmove
   plotatom(x(n),y(n),'w')       % erase the old location by drawing in white
   plotatom(x(n+1),y(n+1),'b')   % plot the new location by drawing in blue
   drawnow
   pause(.1)
   end