\hat{b}^+_i\hat{b}_i\hat{b}^+_j\hat{b}_j=\langle \hat{b}^+_i\hat{b}_i \rangle \hat{b}^+_j\hat{b}_j+\langle \hat{b}^+_j\hat{b}_j \rangle \hat{b}^+_i\hat{b}_i+\langle \hat{b}^+_i\hat{b}^+_j \rangle \hat{b}^+_i\hat{b}_j+\langle \hat{b}^+_i\hat{b}_j \rangle \hat{b}^+_i\hat{b}^+_j +\langle \hat{b}^+_i\hat{b}_j \rangle \hat{b}^+_j\hat{b}_i+\langle \hat{b}^+_j\hat{b}_i \rangle \hat{b}^+_i\hat{b}_j