Multi-configuration character

Computational quantum chemistry has emerged in recent years as a viable tool for the elucidation of molecular structure and molecular properties, especially for the prediction of geometrical parameters, kinetics and thermodynamics of highly labile compounds such as nitrosomethanides. However, they are difficult objects for both experimental (high toxicity, redox lability, high reactivity, explosive character etc.) and computational studies, even with today s sophisticated techniques (e.g. NO compounds are often species with open-shell biradical character which requires the application of multi-configuration methods). 

Vinylmethylene cis- or trans-) (11) is the prototypical a, P-unsaturated carbene. The n system of vinylmethylene allows delocalization of the single electron occupying the p orbital in the triplet and open shell (op) singlet state, resulting in an allyl radical-like tt system. The biradical character of such wavefunctions demands a multi-configuration treatment as artificial symmetry breaking is observed with HF theory. ... 

There is little reason to think the DIP-STEOM or DIP-EOM methods would treat triplet states more accurately than singlet states, as both arise naturally in the deletion of two electrons from the closed-shell reference. We note the multi-configurational nature of the DIP-STEOM states collected in Table III this character presents a significant difficulty to the DFT calculations used in assigning the experimental spectrum. The MCSTEP calculations, on the other hand, provide little treatment of dynamic correlation. The DIP-STEOM and DIP-EOM methods, in contrast, offer an extended treatment of dynamic and non-dynamic correlation, and these results should be considered the best treatment of this collection of vertical IPs so far. 

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