Computation of the Isotropic Hyperfine Coupling Constant: Efficiency and Insights from a New Approach Based on Wave Function Theory

Abstract

The present paper reports an original computational strategy for the computation of the isotropic hyperfine coupling constants (hcc). The algorithm proposed here is based on an approach recently introduced by some of the authors, namely, the first-order breathing orbital self-consistent field (FOBO-SCF). The approach is an almost parameter-free wave function method capable to accurately treat the spin delocalization together with the spin polarization effects while staying in a restricted formalism and avoiding spin contamination. The efficiency of the method is tested on a series of small radicals, among which four nitroxide radicals and the comparison with high-level ab initio methods show very encouraging results. On the basis of these results, the method is then applied to compute the hcc of a challenging system, namely, the DEPMPO-OOH radical in various conformations. The reference values obtained on such a large system allows us to validate a cheap computational method based on density functional theory (DFT). Another interesting feature of the model applied here is that it allows for the rationalization of the results according to a relatively simple scheme based on a two-step mechanism. More precisely, the results are analyzed in terms of two separated contributions: first the spin delocalization and then the spin polarization.

Hyperion: A New Computational Tool for Relativistic Ab Initio Hyperfine Coupling

“…In a comprehensive review of the spin-polarization model for HFC, Chipman refers to second- and higher-order correlation effects / true correlation , thus dividing spin-polarizing single excitations from other excited configurations sampled by FCI. While this split view helps balance computational efficiency with accuracy in multiconfigurational calculations, it has also led to a prevalence in the HFC literature of single-excitation-only results, with no discussion of higher-order effects. , This is likely a safe approximation for systems such as simple organic radicals; however, there is no reason to assume a priori that the SP contribution to HFC is more important than contributions from higher-order excitations. In fact, early work shows that high-level CI approximations are required even for second period atoms to achieve quantitative agreement with experiment .…”

Interplay between Electronic Correlation and Metal–Ligand Delocalization in the Spectroscopy of Transition Metal Compounds: Case Study on a Series of Planar Cu2+ Complexes

“…The perturbation analysis of section suggests that it is possible to define a minimal selected CI, referred to as FOBOCI ,, (first-order breathing orbital CI) that contains the dominant physical effects related to the LMCT determinant. The minimal CI should contain the ROHF and LMCT determinants, together with all single excitations from these two configurations to introduce the necessary orbital relaxation.…”

Integrated Multiscale Multilevel Approach to Open Shell Molecular Systems

“…The resulting unrestricted MLHF (UMLHF) and UMLDFT are able to account for electrostatic (polarization) forces and Pauli repulsion between active and inactive QM regions, whereas long-range electrostatic and polarization terms are effectively taken into account at a low computational cost through the interaction with the FQ layer. To show the potentialities of the resulting UMLHF­(UMLDFT)/MM­(FQ) method, it is challenged to compute hyperfine coupling constants (hccs) of selected molecular spin probes. − Since hccs are particularly sensitive to the probes’ external environment, − they represent an ideal test bed for the novel multiscale multilevel approach.…”