Comparison of diatomics-in-molecules and simple valence-bond potential surfaces for FH2

The method of diatomics-in-molecules (DIM) is applied to the FH₂ system. With spin—orbit interaction neglected, all elements of the 24 × 24 hamiltonian matrix are tabulated as analytic functions of the six diatomic fragment potential curves. It is found that neglect of off-diagonal 8 × 8 blocks in the DIM hamiltonian matrix leads to an energy expression for the ground 1 ²A′ level which is identical to the valence-bond formula used by Blais and Truhlar in dynamical studies of the F + D₂ reaction. The ²A″ excited level from DIM theory is identical to the result derived by Blais and Truhlar, without neglect of the 8 × 8 off-diagonal blocks. The DIM and simple valence-bond energies are compared numerically for noncollinear geometries.