This, in general, is not an easy task, since we are looking for a global minimum and finding the zeroes of the partial derivatives of ε over all α i is not sufficient. If ψ (α) is expressed as a linear combination of other functions (α i being the coefficients), as in the Ritz method, there is only one minimum and the problem is straightforward. There are other, non-linear methods, however, such as the Hartree–Fock method, that are also not characterized by a multitude of minima and are therefore comfortable in calculations.