(\psi \rightarrow \phi) = |\lang \psi |\phi \rang|^2 = |\sum_i\psi^*_i \phi_i |^2 = \sum_{i} |\psi_i^*\phi_i|^2 + \sum_{ij;i \ne j} \psi^*_i \psi_j \phi^*_j\phi_i