In the case of NH3, two reasonable geometries can be tried. Molecular orbitals are the main electronic structural units for analysis and solution of chemical problems at the electronic level and Quantum mechanical description of the changes in electronic structure due to distortions in molecular shape and vice versa is given in the form of the vibronic coupling theory.
The most famous concept based on this theory is the Jahn−Teller (JT) effect. The second-order Jahn-Teller effect (SOJT) is an example of reactions proceeding by an interaction between the HOMO and the LUMO within the same molecule In the high-symmetry regular triangular configuration D3h with the N atom in the center, the ground-state configuration of the system is singlet 1A1, that in the direction coordinate instability of Qa2" with the time-dependent DFT (TD-DFT) calculations symmetry descents and to form a square-pyramidal structure. The intrinsic reaction coordinate (IRC) theory in the present paper is presented for further understanding of the mechanism of such distortion. Natural bond analysis (NBO) is used for illustrating the strongest interaction and natural atomic charges of these structures. The calculated energy profile has been supplemented with optimization by means of transition state theory (TST).