Theoretical study on the core-excited states of the allyl using CVS-icMRCISD method

TL;DR

This paper employs the CVS-icMRCISD computational scheme to accurately simulate the core-excited states of allyl radicals and related species, providing insights that align well with experimental X-ray absorption spectra.

Contribution

It introduces the CVS-icMRCISD method for unbiased, multi-state simulation of core-excited states, improving upon previous approaches that required manual adjustments.

Findings

The simulated XAS spectra match experimental data closely.

The method captures characteristic peaks of allyl radicals and cations.

The approach simplifies multi-state calculations and enhances convergence.

Abstract

The allyl radical (C3H5) is a well-characterized hydrocarbon radical, renowned for its pivotal role as an intermediate species in high-energy environments. Its core excited states can elucidate intricate details pertaining to its electronic and structural properties. The core excited states of allyl were studied experimentally using X-ray absorption spectroscopy (XAS), and the primary characteristic peaks were assigned using the MCSCF approach, but not entirely. In this work, the recently developed CVS-icMRCISD scheme was used to simulate the excitation and ionization processes of C's K-shell electrons within allyl radicals, cations, and anions, respectively. Our results indicate that the XAS spectrum obtained not merely captured the distinctive peaks associated with allyl radicals, but also encompassed the characteristic peaks pertaining to allyl cations. Meanwhile, unlike manually adjusting the state weights of different electronic states to align with experimental spectral data, we adopt the CVS-icMRCISD scheme, which uses state averaging and produces unbiased results, making it suitable for studying multiple states simultaneously and easy to converge. More importantly, when accounting for the dynamic electron correlation, our results align seamlessly with the experimental XAS. This congruence underscores the potential of our CVS-icMRCISD as a robust tool for theoretical investigations pertaining to the excitation of inner shell electrons in small molecules.