Contact interaction between two box modes ($I_{1111}=I_{2222}=3/(2L),I_{1122}=1/L$)

For distinct box modes ${\phi }_1=\sqrt{2/L}\sin (p\pi x/L)$ and ${\phi }_2=\sqrt{2/L}\sin (q\pi x/L)$, the displayed contact interaction overlaps produce self terms $3g{\sum }_j{n}_j(n_j-1)/(4L)$, a density term $2g{n}_1{n}_2/L$, and pair conversion $g(a_1^{†2}{a}_2^2+a_2^{†2}{a}_1^2)/(2L)$. Pair conversion preserves total particle number but prevents diagonalization by individual occupation numbers. The additional overlaps are $I_{1112}=-{\delta }_{q,3p}/(2L)$ and $I_{1222}=-{\delta }_{p,3q}/(2L)$, following from ${\sin }^{3}u=(3\sin u-\sin 3u)/4$. Third-harmonic mode pairs therefore also permit single-particle transfer assisted by the other particles. For the first two box modes these additional terms vanish.