Energy from schrödinger equation

Gavroy
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hi

i was currently thinking about this step here:

\langle \psi_n, - \Delta \psi_n \rangle = \int |\nabla \psi_n|^2 dx

how do you get from the laplacian to this other expression?
 
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Integrate by parts to move a del operator from the right psi to the left one.
 

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