variational parameters in the GWF are obtained as follows: Starting from \(|\Psi_0\rangle\) and \(\hat{P}_{\rm G}\ ,\) the variational energy \(E_{\rm var}=\langle\hat{H}\rangle\) for a given model Hamiltonian , e.g., for the Hubbard model (read a /historical note on its invention), must be calculated. Then, \(E_{\rm var}\) must be minimized with respect to