Multiple conical intersections in small linear parameter Jahn-Teller systems: the DMBE potential energy surface of ground-state C3 revisited

Accurate CHIPR Potential Energy Surface for the Lowest Triplet State of C3

We report the first global ab initio-based potential energy surface (PES) for ground-state triplet C₃(³A') based on accurate energies extrapolated to the complete basis set (CBS) limit, and using the combined-hyperbolic-inverse-power-representation method for the analytical modeling. By relying on a cost-effective CBS(D,T) protocol, we ensure that the final form reproduces all topographical features of the PES, including its cyclic-linear isomerization barrier, with CBS(5,6)-quality. To partially account for the incompleteness of the N-electron basis and other minor effects, the available accurate experimental data on the relevant diatomics were used to obtain direct-fit curves that replace the theoretical ones in the many-body expansion. Besides describing properly long-range interactions at all asymptotic channels and permutational symmetry by built-in construction, the PES reported here reproduces the proper exothermicities at dissociation regions as well as the spectroscopy of the diatomic fragments. Bound vibrational state calculations in both linear and cyclic isomers have also been carried out, unveiling a good match of the available data on C₃(ã ³Π_u), while assisting with IR band positions for C₃(³A₂^') that may serve as a guide for its laboratory and astronomical detection.

Difficulties and Virtues in Assessing the Potential Energy Surfaces of Carbon Clusters via DMBE Theory: Stationary Points of Cκ (κ = 2-10) at the Focal Point

The previously reported potential energy surfaces (PESs) of ground-state singlet C₃ and triplet C₄ are here utilized as input for the construction of approximate cluster expansions for larger C_κ (κ = 5-10) species. Relying primarily on the double many-body expansion (DMBE) approach, global potentials are obtained by summing the total interaction energies of all atomic subclusters up to four-body terms. The notable capability of the final forms in predicting good estimates of the linear global minima and their thermochemical/structural properties may provide important insights into the structure-determining nature of the (2 + 3 + 4) terms. The main difficulties and virtues in assessing C_κ's via DMBE theory are analyzed and new prospects given for the construction of global reliable PESs for the target species.

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.