The above dispersive Gaussian wave packet, unnormalized and just centered at the origin, instead, at t =0, can now be written in 3D, now in standard units: ψ ψ ( r , 0 ) = e − − r ⋅ ⋅ r / 2 a , where a is a positive real number, the square of the width of the wave packet, a = 2 ⟨ ⟨ r ⋅ ⋅ r ⟩ ⟩ / 3 ⟨ ⟨ 1 ⟩ ⟩ = 2 ( Δ Δ x ) 2 .