Ab initio derivation of multi-orbital extended Hubbard model for molecular crystals

TL;DR

This study derives a multi-orbital extended Hubbard model for molecular crystals from ab initio CI calculations, capturing inter-molecular interactions and band structure features, and introduces a fragment-based approach for effective modeling.

Contribution

It presents a novel ab initio method to derive multi-orbital Hubbard models for molecular crystals, including inter-molecular interactions and a fragment decomposition strategy.

Findings

The model reproduces the CI Hamiltonian matrix for isolated molecules.

Band structure analysis confirms overlapping and mixing of two bands.

Fragment decomposition reveals zig-zag two-leg ladder structure.

Abstract

From configuration interaction (CI) ab initio calculations, we derive an effective two-orbital extended Hubbard model based on the gerade (g) and ungerade (u) molecular orbitals (MOs) of the charge-transfer molecular conductor (TTM-TTP)I_3 and the single-component molecular conductor [Au(tmdt)_2]. First, by focusing on the isolated molecule, we determine the parameters for the model Hamiltonian so as to reproduce the CI Hamiltonian matrix. Next, we extend the analysis to two neighboring molecule pairs in the crystal and we perform similar calculations to evaluate the inter-molecular interactions. From the resulting tight-binding parameters, we analyze the band structure to confirm that two bands overlap and mix in together, supporting the multi-band feature. Furthermore, using a fragment decomposition, we derive the effective model based on the fragment MOs and show that the staking TTM-TTP molecules can be described by the zig-zag two-leg ladder with the inter-molecular transfer integral being larger than the intra-fragment transfer integral within the molecule. The inter-site interactions between the fragments follow a Coulomb law, supporting the fragment decomposition strategy.