Once the state vector was introduced, matrix mechanics could be rotated to any basis, where the H matrix need no longer be diagonal. The Heisenberg equation of motion in its original form states that A mn evolves in time like a Fourier component, A m n ( t ) = e i ( E m − − E n ) t A m n ( 0 ) , which can be recast in differential form d A m n d t = i ( E m − − E n ) A m n , and it can be restated so that it is true in an arbitrary basis, by noting that the H matrix is diagonal with diagonal values E m, d A d t = i ( H A − − A H ) . This is now a matrix equation, so it holds in any basis. This is the modern form of the Heisenberg equation of motion.