Multistate density function theory and the construction of diabatic and adiabatic potential energy surfaces

A multistate density function theory (MSDFT) based on valence bond theory was introduced. As an application, the MSDFT method was illustrated by the bond dissociation of H₂ and the proton transfer between HNO₃ and a water molecule. In the dissociation of H₂, the MSDFT method yields a correct behavior as the two H atoms stretch to infinity, and gives a potential well in accord with second-order perturbation using complete active space (CASPT2). For the proton transfer process of HNO₃, MSDFT can be used to yield both diabatic and adiabatic potential energy curves as a function of the proton transfer reaction coordinate. For the reaction barrier height, the inclusion of an ionic state in a three-state model can significantly improve the accuracy in barrier height in comparison with the high-level results.