where ∇ is the gradient operator, · is the dot product, ∂ ∂ f ∂ ∂ p = e ^ ^ x ∂ ∂ f ∂ ∂ p x + e ^ ^ y ∂ ∂ f ∂ ∂ p y + e ^ ^ z ∂ ∂ f ∂ ∂ p z = ∇ ∇ p f is a shorthand for the momentum analogue of ∇, and ê x, ê y, ê z are Cartesian unit vectors.