where N det is the number of determinants. For sake of conciseness, we will restrict the discussion to the ground state (i.e. k = 0) and drop the subscript k accordingly. Solving the eigenvalue problem in this basis is referred to as FCI and yields, for a given basis set, the exact solution of the Schrödinger equation. Unfortunately, FCI is usually computationally intractable because of its exponential scaling with the size of the system.