Optimization of diatomic state mixing in diatomics-in-molecules theory: The CHn potential-energy surfaces

The diatomics-in-molecules (DIM) theory is extended to include larger valence bond (VB) basis sets and the optimization of mixing between the diatomic VB structures. This mixing is essential to the calculation of DIM potential-energy hypersurfaces (PEH) using basis sets large enough to generate more than one diatomic state of a given symmetry. In keeping with the philosophy of DIM, the diatomic state mixing is treated as being independent of the number of atoms in the molecule. The method is used to calculate ground and excited state potential energies for CH2, CH3, and CH4. These extended DIM results demonstrate a number of improvements over earlier DIM methods and are in good agreement with ab initio and experimental results.