An externally corrected coupled cluster (CC) method, where an adaptive configuration interaction (ACI) wave function provides the external cluster amplitudes, named ACI-CC, is presented. By exploiting the connection between configuration interaction and CC through cluster analysis, the higher-order T 3 and T 4 terms obtained from ACI are used to augment the T 1 and T 2 amplitude equations from traditional CC. These higher-order contributions are kept frozen during the CC iterations and do not contribute to an increased cost with respect to coupled cluster including the single and double excitations (CCSD). We have benchmarked this method on three closed-shell systems: beryllium dimer, carbonyl oxide, and cyclobutadiene, with good results compared to other corrected CC methods. In all cases, the inclusion of these external corrections improved upon the "gold standard" CCSD(T) results, indicating that ACI-CCSD(T) can be used to assess strong correlation effects in a system and as an inexpensive starting point for more complex external corrections.
The methylene amidogen radical (HCN) plays a role in high-energy material combustion and extraterresterial atmospheres. Recent theoretical work has struggled to match experimental assignments for its CN and antisymmetric CH stretching frequencies (ν and ν), which were reported to occur at 1725 and 3103 cm. Herein, we compute the vibrational energy levels of this molecule by extrapolating quadruples-level coupled-cluster theory to the complete basis limit and adding corrections for vibrational anharmonicity. This level of theory predicts that ν and ν should occur at 1646 and 2892 cm, at odds with the experimental assignments. To investigate the possibility of defects in our theoretical treatment, we analyze the sensitivity of our approach to each of its contributing approximations. Our analysis suggests that the observed deviation from experiment is too large to be explained as an accumulation of errors, leading us to conclude that these transitions were misassigned. To help resolve this discrepancy, we investigate possible byproducts of the H + HCN reaction, which was the source of HCN in the original experiment. In particular, we predict vibrational spectra for cis-HCNH, trans-HCNH, and HCNH using high-level coupled-cluster computations. Based on these results, we reassign the transition at 1725 cm to ν of trans-HCNH, yielding excellent agreement. Supporting this identification, we assign a known contaminant peak at 886 cm to ν of the same conformer. Our computations suggest that the peak observed at 3103 cm, however, does not belong to any of the aforementioned species. To facilitate further investigation, we use structure and bonding arguments to narrow the range of possible candidates. These arguments lead us to tentatively put forth formaldazine [(HCN)] as a suggestion for further study, which we support with additional computations.
High level ab initio methods are employed to study the addition of methanol to the simplest Criegee intermediates and its methylated analogue. Kinetic rate constants over a range of temperatures are computed and compared to experimental results.
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