[Paper Review] Molecular Transport

María Camarasa‐Gómez, Daniel Hernangómez‐Pérez+3 more|arXiv (Cornell University)|Nov 3, 2024

Ferrocene Chemistry and ApplicationsChemistry3 citations

TL;DR

This paper presents AITRANSS, an ab initio post-processing package integrated with FHI-aims for simulating electronic transport in single-molecule junctions using the non-equilibrium Green's function formalism. It enables accurate calculation of transmission, density of states, spin properties, and STM images, with support for spin-orbit coupling and self-consistent non-equilibrium DFT, offering a flexible and efficient tool for studying quantum transport in nanoscale systems.

ABSTRACT

Single-molecule junctions - nanoscale systems where a molecule is connected to metallic electrodes - offer a unique platform for studying charge, spin and energy transport in non-equilibrium many-body quantum systems, with few parallels in other areas of condensed matter physics. Over the past decades, these systems have revealed a wide range of remarkable quantum phenomena, including quantum interference, non-equilibrium spin-crossover, diode-like behavior, or chiral-induced spin selectivity, among many others. To develop a detailed understanding, it turned out essential to have available ab initio-based tools for accurately describing quantum transport in such systems. They need to be capable of capturing the intricate electronic structure of molecules, sometimes in the presence of electron-electron or electron-phonon interactions, in out-of-equilibrium environments. Such tools are indispensable also for experimentally observed phenomena explained in terms of parametrized tight-binding models for the quantum transport problem. While FHI-aims also offers specialized transport routines, e.g. for chemically functionalized nanotubes or nanotube networks, our focus in this section is on the AITRANSS package designed for simulations of single-molecule transport. AITRANSS is an independent post-processing tool that combined with FHI-aims enables the calculation of electronic transport properties, as well as atom-projected density of states, spin properties and the simulation of scanning tunneling microscope images in molecular junctions. Pilot versions of the code extend some of these capabilities to non-linear transport in the applied bias, with plans to include these features in future releases of the package.

Motivation & Objective

• To develop a robust, ab initio tool for simulating electronic transport in single-molecule junctions under non-equilibrium conditions.
• To enable accurate modeling of quantum transport phenomena such as quantum interference, spin selectivity, and diode-like behavior.
• To provide a flexible and efficient computational framework compatible with FHI-aims for both post-processing and self-consistent non-equilibrium DFT calculations.
• To extend capabilities to include spin-orbit coupling, current-induced forces, and future multiterminal and dynamical transport simulations.
• To improve accuracy of Kohn-Sham spectral properties through scissor-like corrections and hybrid functionals.

Proposed method

• Uses the non-equilibrium Green’s function (NEGF) formalism to compute the ballistic transmission function via the trace formula involving the Green’s function and lead self-energies.
• Employs a model self-energy approach with absorbing boundary conditions using energy-independent real and imaginary parts (δε and η) to simulate electron leakage from the scattering region.
• Reconstructs the Kohn-Sham Hamiltonian from FHI-aims output using Löwdin orthogonalization to handle non-orthogonal basis sets.
• Supports both post-processing-only and self-consistent feedback loops between FHI-aims and AITRANSS, ensuring consistent non-equilibrium density matrices and Fermi level adjustment.
• Implements parallelization via OpenMP and plans to extend to MPI for improved scalability, with future migration to HDF5 for data storage.
• Uses FHI-aims output files (basis.out, omat.aims, mos.aims) as input, with specialized handling for spin-polarized and spin-orbit-coupled systems.

Experimental results

Research questions

• RQ1 How can ab initio quantum transport simulations be efficiently and accurately performed in single-molecule junctions with complex electronic and spin degrees of freedom?
• RQ2 What is the role of self-consistency in non-equilibrium DFT for correctly describing charge transport and Fermi level alignment?
• RQ3 How can spin-orbit coupling and spin-selective transport be reliably modeled in molecular junctions using first-principles methods?
• RQ4 What improvements can be made to Kohn-Sham spectral properties using scissor-like corrections and hybrid functionals in transport calculations?
• RQ5 How can the simulation framework be extended to include multiterminal geometries, current-induced forces, and light-matter interactions?

Key findings

• AITRANSS successfully implements the NEGF formalism with accurate treatment of self-energies and absorbing boundary conditions, enabling reliable transmission function calculations.
• The package supports both closed-shell and spin-polarized systems, including spin-orbit coupling, with validated results for molecular junctions.
• Self-consistent non-equilibrium DFT cycles are implemented via an external shell script, maintaining performance for small to medium molecular junctions.
• The code is parallelized using OpenMP and is integrated into the FHI-aims build system, ensuring reproducibility and ease of use.
• Future developments include MPI parallelization, HDF5 data format migration, and enhanced accuracy via hybrid functional-based scissor corrections.
• The framework enables simulation of scanning tunneling microscope images and current-induced forces, with plans to extend to terahertz light coupling and multiterminal systems.