Ab Initio Adiabatic Potential Energy Surfaces and Non-adiabatic Couplings for O₃: Construction of Four State Diabatic Hamiltonian

We compute highly accurate first principle based ab initio adiabatic potential energy surfaces (PESs) using State-Averaged Multi-Configurational Self-Consistent Field (SA-MCSCF) followed by internally contracted Multi-Reference Configuration Interaction method incorporating fixed-reference Davidson corrections [ic-MRCI(Q)], where a full valence active space of 18 electrons in 12 orbitals and aug-cc-pVQZ basis set are employed for the low-lying four singlet electronic states of ozone ${X}^1A'$, $1 ^1A"$, $1 ^1A'$ and $2^1A"$. It accurately reproduces the dissociation energies of ozone (1.101 eV) as well as the molecular oxygen (5.106 eV) along with vibrational frequencies of O₃ in comparison with experimental data. To ensure appropriate accuracy and proper convergence in the interaction as well as asymptotic regions, we (a) extend the number of electronic states in SA-MCSCF calculation (singlet as well as triplet and quintet); (b) systematically expand the active space \[(12e,9o) → (18e,12o) → (24e,15o)\] and basis set size AVDZ $→$ AV6Z $→$ Complete Basis Set limit; (c) incorporate multi-reference character along with Davidson correction. Conical intersections between the adjacent electronic states (1-2, 2-3 and 3-4) are located at C_2v, D_3h as well as C_s geometries through the four-state adiabatic-to-diabatic transformation of non-adiabatic coupling terms (NACTs) computed at Coupled-Perturbed Multi-Configurational Self-Consistent Field (CP-MCSCF) method along the circular contours. Finally, we present: (a) ic-MRCI(Q) calculated minimum energy path of incoming oxygen to the diatom O$₂$ is devoid of any "reef" feature; (b) NACTs and diabatic PES matrix elements as function of hyperangles (θ,φ) at a fixed hyperradius ρ= 4 Bohr for a four state sub-Hilbert space.