To start, a classical turning point, x 1 is chosen and 2 m ℏ ℏ 2 ( V ( x ) − − E ) is expanded in a power series about x 1 : 2 m ℏ ℏ 2 ( V ( x ) − − E ) = v 1 ( x − − x 1 ) + v 2 ( x − − x 1 ) 2 + ⋯ ⋯