Ab initio potential energy function and geometry of the A ̃ state of ammonia

A model potential energy function of the A ̃ state of NH₃ was determined using the multiconfiguration linear response approach. From the model potential function and its simple empirical modification, the energies and effective geometries of the 〈v₁, v₂, v₃ = 0, v₄, l₄; J, k| quasi-bound states were predicted for ¹⁴NH₃ and ¹⁴ND₃ using the nonrigid inverter theory. For the out-of-plane bending states 〈v₁ = 0, v₂, v₃ = 0, v₄ = 0; J, k| the predicted values agree (reasonably for the pure ab initio function, nearly quantitatively for its empirical modification) with available experimental data, indicating adequacy of the used theory for studying low-lying quasi-bound states of ammonia in its A ̃ electronic state.