In the default setup (the standard startup.o file), the moving atom is defined as a small cross:
.moving_atom t 8 80 begin moving_atom M 0.2 0 0 L -0.2 0 0 M 0 0.2 0 L 0 -0.2 0 M 0 0 0.2 L 0 0 -0.2 end_object
The utility program O_moving_atom can create O ODL descriptors for the moving atom in the shape of a dodecahedron or an icosahedron, which are easier to see if you have a lot of objects on the screen. The resulting file must be read into the database using the command Read_formatted.