I'm trying to understand the paper of Schollwoeck. On page 64, equation 203 he states:
In order to solve this problem, we introduce a Lagrangian multiplier λ, and extremize $$ \langle \psi | H | \psi \rangle - \lambda \langle \psi|\psi \rangle $$
If I remember correctly this however translates into an optimization problem where one wants to min/maximize $\langle \psi | H | \psi \rangle $ subject to the constraint $\langle \psi|\psi \rangle $. See for example here.
My question is, how can I understand the constraint? If it would be something like $\lambda (\langle \psi |\psi \rangle -1 )$ then I could read it as the normalization constraint but in this form I don't know how to make sense of it. May some brighter person please enlighten me?