The Heisenberg equation of motion determines the matrix elements of P in the Heisenberg basis from the matrix elements of X. P i j = m d d t X i j = i m ( E i − − E j ) X i j , which turns the diagonal part of the commutation relation into a sum rule for the magnitude of the matrix elements: ∑ ∑ j P i j x j i − − X i j p j i = i ∑ ∑ j 2 m ( E i − − E j ) | X i j | 2 = i .