From my understanding of quantum mechanics, I know that it is difficult to support an understandable answer to this question.

In all physics textbooks it is written that the Psi wave function and its first derivative must be continuous.

Meanwhile, in these same books you find many situations where the Psi solution is continuous and its derivative is not, the simplest example is the particle in a one-dimensional box [1].

My own explanation is that Psi is a complex mathematical function that only has physical meaning as the product of Psi. Psi* represents the probability density of finding a particle and therefore:

Psi.Psi* and its derivative must be continuous while the derivative of Psi alone may or may not.

Let's take the simple example of real Psi "Psi = Psi *" with fixed impenetrable limits where Psi = 0

It is to be expected that the “probability function” Psi^2 and its derivative are continuous.

It follows that the derivative of Psi is continuous if Psi is not equal to zero.

Which is probably missing in the literature.

1-Quora Questions/Answers, 2021.

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