Potential energy of the molecule as a function of the displacements of the atoms from equilibrium positions. The harmonic terms are quadratic in the displacements and the anharmonic terms are of cubic, quartic and higher orders in the displacements. $Q$ are normal coordinates $$2V=\sum _k{\lambda }_k{Q}_k^2+\sum _{ijk}{k}_{ijk}{Q}_i{Q}_j{Q}_k+higherorderterms$$ In terms of internal coordinates, $$2V=\sum _i{F}_{ii}{R}_i^2+\sum _{i,j\ne i}{F}_{ij}{R}_i{R}_j+\sum _{i,j,k}{F}_{ijk}{R}_i{R}_j{R}_k+higherorderterms$$