Steric clashes (pink dotted lines)
develop during rotation around φ and ψ.
Real atoms are much larger than these balls (see below).

Rotation around Phi (&phi)
Rotation around Psi (&psi)

Sizes of real atoms (translucent).
Drag on the model to rotate it.

(Credits: Original Page -- Animations by David Sanders and Jim Naismith (used with permission) -- Jmol added by Eric Martz)