Calculating Hyperfine Couplings in Large Ionic Crystals Containing Hundreds of QM Atoms: Subsystem DFT is the Key

TL;DR

This paper demonstrates that subsystem DFT with linear scaling can accurately compute hyperfine couplings in large ionic crystals, overcoming limitations of smaller models and QM/MM methods.

Contribution

The work introduces the application of linear scaling FDE subsystem DFT to large ionic crystals, enabling accurate hyperfine coupling calculations for systems with thousands of atoms.

Findings

Large QM regions are necessary to capture embedding effects.

FDE approach effectively handles systems with hundreds of QM atoms.

Accurate hyperfine couplings achieved for 15,000-atom system.

Abstract

We present an application of the linear scaling Frozen Density Embedding (FDE) formulation of subsystem DFT to the calculation of isotropic hyperfine coupling constants (hfccs) of atoms belonging to a guanine radical cation embedded in a guanine hydrochloride monohydrate crystal. The model systems considered range from an isolated guanine to a 15,000 atom QM/MM cluster where the QM region is comprised of 36 protonated guanine cations, 36 chlorine anions and 42 water molecules. Our calculations show that the embedding effects of the surrounding crystal cannot be reproduced neither by small model systems nor by a pure QM/MM procedure. Instead, a large QM region is needed to fully capture the complicated nature of the embedding effects in this system. The unprecedented system size for a relativistic all-electron isotropic hfccs calculation can be approached in this work because the local nature of the electronic structure of the organic crystals considered is fully captured by the FDE approach.