Numerical recipes

852 Chapter 19. Partial Differential Equations
conditions that ψ → 0 at x →±∞. Suppose we content ourselves with firstorder
accuracy in time, but want to use an implicit scheme, for stability. A slight
generalization of (19.2.8) leads to
i
for which
�
n+1
ψj − ψn j
∆t
�
= −
�
n+1
ψj+1 − 2ψn+1 j
(∆x) 2
+ ψn+1 j−1
�
+ Vjψ n+1
j
(19.2.27)
1
ξ = � � � � (19.2.28)
4∆t k∆x
1+i sin2 + Vj∆t
(∆x) 2 2
This is unconditionally stable, but unfortunately is not unitary. The underlying
physical problem requires that the total probability of finding the particle somewhere
remains unity. This is represented formally by the modulus-square norm of ψ
remaining unity:
� ∞
|ψ| 2 dx =1 (19.2.29)
−∞
The initial wave function ψ(x, 0) is normalized to satisfy (19.2.29). The Schr ödinger
equation (19.2.26) then guarantees that this condition is satisfied at all later times.
Let us write equation (19.2.26) in the form
where the operator H is
i ∂ψ
∂t
The formal solution of equation (19.2.30) is
= Hψ (19.2.30)
H = − ∂2
+ V (x) (19.2.31)
∂x2 ψ(x, t) =e −iHt ψ(x, 0) (19.2.32)
where the exponential of the operator is defined by its power series expansion.
The unstable explicit FTCS scheme approximates (19.2.32) as
ψ n+1
j =(1− iH∆t)ψn j (19.2.33)
where H is represented by a centered finite-difference approximation in x. The
stable implicit scheme (19.2.27) is, by contrast,
ψ n+1
j =(1+iH∆t)−1 ψ n j (19.2.34)
These are both first-order accurate in time, as can be seen by expanding equation
(19.2.32). However, neither operator in (19.2.33) or (19.2.34) is unitary.
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