# Compound-tunable embedding potential method and its application to fersmite crystal

## TL;DR

The paper introduces the compound-tunable embedding potential (CTEP) method for modeling chemical compounds, demonstrating its effectiveness on fersmite crystal and highlighting its potential for studying defects involving f- and d-elements.

## Contribution

A novel CTEP method for embedding potentials is proposed, enabling accurate fragment calculations within periodic crystals, especially for f- and d-element compounds.

## Key findings

- High agreement of electronic density with periodic calculations
- Accurate interatomic distances within the fragment
- Potential for studying defects with relativistic effects

## Abstract

Compound-tunable embedding potential (CTEP) method is proposed. A fragment of some chemical compound, "main cluster" in the present paper, is limited by boundary anions such that the nearest environmental atoms are cations. The CTEP method is based on constructing the embedding potential as linear combination of short-range "electron-free" spherical "tunable" pseudopotentials for cations from nearest environment of the main cluster, whereas the long-range CTEP part consists of Coulomb potentials from optimized fractional point charges centered on both environmental cations and anions. A pilot application of the CTEP method to the fersmite crystal, CaNb$_2$O$_6$, is performed and a remarkable agreement of the electronic density and interatomic distances within the fragment with those of the original periodic crystal calculation is attained. Characteristics of "atoms-in-compounds" which are of great importance for compound of $f$- and $d$-elements (Nb in fersmite) are considered on examples of chemical shifts of $K_{{\alpha}_{1,2}}\ $ and $K_{{\beta}_{1,2}}\ $ lines of X-ray emission spectra in niobium. A very promising potential of this approach in studying variety of properties of point defects containing $f$- and heavy $d$-elements with relativistic effects, extended basis set and broken crystal symmetry considered is discussed.