Combined Modelling of Triply Paired Electronic States of HO2 + ion with 3 Symmetry using Coupled Eigen Model.

The multi state triply paired coupled adiabatic electronic states of X 3 A ' ' ${X\,^3 A^{\prime \prime } }$ 1 3 A ' ' ${1\,^3 A^{\prime \prime } }$ , 2 3 A ' ' ${2\,^3 A^{\prime \prime } }$ symmetries for the title ion have been modelled using the triply paired coupled eigen valued functions with the principal aim of getting a set of well optimized triply paired coupled functions, capable of explaining all the microscopic topograhical features present in the chosen coupled electronic states. The high quality size truncated dynamically correlated points at the limit of complete basis set (CBS) were utilized for this purpose. The proposed model function are presented in an "easy to understand" linear form with respect to the pre-defined variables which are directly dependent on the diabatic potentials and coupling strengths found in the diabatic matrix. Its most important feature (an essential criterion to be selected for modelling the triply degenerate seam pathway) is the ability to predict the existence of an uniquely defined triply degenerate iso-energetic geometry among the chosen coupled states in addition to the prediction of the existence of many doubly degenerate iso-energetic geometries in those states and hence can be adaptable to any general molecular system where such triply or doubly degenerate geometries are believed to occur. The title ion is one among them where many doubly iso-energetic geometries were located but not the triply degenerate one as it is a three atomic system unlike the polyatomic case where the triply degenerate geometry will most likely to be found. To do this work, one requires the knowledge of four asymptotic diatomic curves upon atom+diatom dissociation whose correct correlated spectroscopic states are O2 + ( X 2 Π g ) ${(X\,^2 {\rm{\Pi }}_g )}$ , O2 ( X 3 Σ g - ) ${(X\,^3 {\rm{\Sigma }}_g^ - )}$ , O2 + ( a 4 Π u ) ${(a\,^4 {\rm{\Pi }}_u )}$ , OH+ ( X 3 Σ - ) ${(X\,^3 {\rm{\Sigma }}^ - )}$ . The quality and the characteristics of analytical surface of the ion is discussed well in detail.

Role of Augmented Basis Sets and Quest for ab Initio Performance/Cost Alternative to Kohn-Sham Density Functional Theory.

Following a previous work, we have assessed the feasibility of MP2/CBS(d, t) as an alternative to state-of-the-art density functionals. The effect of using augmented basis sets is here tested on the 76 barrier heights and 10 isomerization reactions previously utilized. Moreover, calculations for 20 sets of the GMTKN24 database for thermochemistry, kinetics, and noncovalent interactions have been performed. For the density functional theory calculations, M06-2X and B3LYP-D3 functionals are utilized as two representative functionals, while MP2 and CCSD(T) methods are employed as the ab initio counterparts. The results show that MP2 calculations perform similarly to the ones obtained with M06-2X insofar as accuracy and computational cost are concerned. For all methods, the use of augmented basis sets yields enhanced results for anionic systems when compared with the ones from non-augmented bases. Otherwise, the basis-set change effect is found to be minimal. It is therefore concluded that the use of large basis sets is unjustified when facing the increase in computational cost.

Global Potential Energy Surface for HO2+ Using the CHIPR Method.

An analytical potential energy function for the title ion based on the combined hyperbolic inverse power representation (CHIPR) method and its characteristics are discussed at length in the present work. The curves of two diatomic ions, O2+ and OH+, are also obtained within the same approach. The model PES so obtained exhibits extraordinary flexibility in describing with subchemical accuracy even the weak topological features near the higher energy regions. Thus, structural properties predicted by the model may help spectroscopists who want to compare their experimental values with the ones from theory. The relaxed PESs in various coordinates have been calculated by relaxing the O2 bond distance using the present model, thus throwing light on all the possible isomers and their interconversions. The latest estimates of IR frequencies for three vibrational modes have been compared with the computed frequencies using the present model, and the agreement seems encouraging.

Ab initio study of the O4H(+) novel species: spectroscopic fingerprints to aid its observation.

A detailed ab initio characterization of the structural, energetic and spectroscopic properties of the novel O4H(+) species is presented. The equilibrium structures and relative energies of all multiplet states have been determined systematically by analyzing static and dynamical correlation effects. The two and three body dissociation processes have been studied and indicate the presence of conical intersections in various states including the ground state. Comparison with available thermochemical data is very good, supporting the applied methodology. The reaction, H3(+) + O4→ O4H(+) + H2, was found to be exothermic ΔH = -19.4 kcal mol(-1) and therefore, it is proposed that the product in the singlet state could be formed in the interstellar medium (ISM) via collision processes. To aid in its laboratory or radioastronomy detection in the interstellar medium we determined spectroscopic fingerprints. It is estimated for the most stable geometry of O4H(+) dipole allowed electronic transitions in the visible region at 429 nm and 666 nm, an intense band at 1745 cm(-1) in the infrared and signals at 40.6, 81.2 and 139.2 GHz in the microwave region at 10, 50 and 150 K respectively, relevant for detection in the ISM.

Ab initio adiabatic and quasidiabatic potential energy surfaces of lowest four electronic states of the H+ + O2 system.

Ab initio global adiabatic and quasidiabatic potential energy surfaces of lowest four electronic (1-4 (3)A(")) states of the H(+)+O(2) system have been computed in the Jacobi coordinates (R,r,γ) using Dunning's cc-pVTZ basis set at the internally contracted multireference (single and double) configuration interaction level of accuracy, which are relevant to the dynamics studies of inelastic vibrational and charge transfer processes observed in the scattering experiments. The computed equilibrium geometry parameters of the bound [HO(2)](+) ion in the ground electronic state and other parameters for the transition state for the isomerization process, HOO(+)⇌OOH(+) are in good quantitative agreement with those available from the high level ab initio calculations, thus lending credence to the accuracy of the potential energy surfaces. The nonadiabatic couplings between the electronic states have been analyzed in both the adiabatic and quasidiabatic frameworks by computing the nonadiabatic coupling matrix elements and the coupling potentials, respectively. It is inferred that the dynamics of energy transfer processes in the scattering experiments carried out in the range of 9.5-23 eV would involve all the four electronic states.

Nonadiabatic dynamics on the two coupled electronic PESs: the H+ + O2 system.

Multistate adiabatic and diabatic PESs were computed for the H+ + O2 collision system in Jacobi coordinates, (R,r,γ) using the cc-pVTZ basis set and the ic-MRCI level of theory. In addition, all possible interaction potentials and nonadiabatic coupling matrix elements among those different electronic states were also computed. Comparisons with earlier computed interaction potentials were made wherever possible, and the differences between them is attributed to the multistate diabatization and the chosen level of theory and basis set. Focusing our attention on the ground-state (GS) and the first excited-state (ES) PES, quantum dynamics were performed using the 2 × 2 diabatic potential submatrix obtained from the multistate (four) diabatic potential matrix within the VCC-RIOSA scheme at two experimentally reported collision energies, E(cm) = 9.5 and 23 eV. The scattering quantities were computed for two experimentally observed collision processes, namely, the inelastic vibrational excitation (IVE), H+ + O2 (X3Σg(−),v = 0) → H+ + O2 (X3Σg(−),v'), and the vibrational charge transfer (VCT), H+ + O2 (X3Σg(−),v = 0) → H (2S) + O (X2Πg,v''). Comparisons were made with experimental results and found an overall improvement relative to the earlier computed results, and the discrepancies, if any, could be brought down to minimum by further modification in employed ab initio PESs and the interaction potential.