Bad Qubits — Learn Quantum by Doing

Multiple choice

DMRG finds ground states by exploiting the variational principle. When it optimizes a single site tensor with all neighboring tensors held fixed, minimizing the energy reduces to which kind of standard problem?

An eigenvalue problem (solving H_eff A = E A for the lowest eigenvalue)
A linear system of equations with no eigenvalues involved
A nonlinear optimization over all tensors simultaneously
A random sampling problem solved by Monte Carlo