[Paper Review] Coulomb Interactions between Dipolar Quantum Fluctuations in van der Waals Bound Molecules and Materials

Martin Stöhr, Mainak Sadhukhan+3 more|arXiv (Cornell University)|Jul 24, 2020

Quantum and electron transport phenomena67 references38 citations

TL;DR

This paper introduces and quantifies 'Coulomb Singles' (CS), a previously neglected many-body contribution to van der Waals (vdW) interactions arising from long-range Coulomb coupling between dipolar quantum fluctuations in confined molecular and nanomaterial systems. Using a perturbative extension of the Many-Body Dispersion (MBD) framework, the authors show CS contributions can reach up to 6 kJ/mol and qualitatively alter long-range vdW interactions, resolving discrepancies in experimental observations of collective behavior in nanostructures.

ABSTRACT

Mutual Coulomb interactions between electrons lead to a plethora of interesting physical and chemical effects, especially if those interactions involve many fluctuating electrons over large spatial scales. Here, we identify and study in detail the Coulomb interaction between dipolar quantum fluctuations in the context of van der Waals complexes and materials. Up to now, the interaction arising from the modification of the electron density due to quantum van der Waals interactions was considered to be vanishingly small. We demonstrate that in supramolecular systems and for molecules embedded in nanostructures, such contributions can amount to up to 6 kJ/mol and can even lead to qualitative changes in the long-range vdW interaction. Taking into account these broad implications, we advocate for the systematic assessment of so-called Coulomb singles in large molecular systems and discuss their relevance for explaining several recent puzzling experimental observations of collective behavior in nanostructured materials.

Motivation & Objective

• To identify and quantify a neglected many-body contribution to van der Waals interactions arising from Coulomb coupling between dipolar quantum fluctuations.
• To address the failure of standard dipole-based models in explaining experimental observations of collective behavior in nanostructured materials.
• To develop a practical, many-body approach within the MBD framework to systematically assess these higher-order multipole contributions.

Proposed method

• Extends the Many-Body Dispersion (MBD) formalism by applying first-order perturbation theory to the full Coulomb interaction beyond the dipole approximation.
• Derives the Coulomb Singles (CS) energy correction as the first-order change in the system's energy due to full Coulomb coupling between fluctuating electron densities.
• Uses effective Drude oscillators to model electron density fluctuations and computes the energy difference between dipole-coupled and non-interacting systems.
• Splits the CS energy into two components: classical electrostatic energy change due to polarization and a correlation correction to dipole-dipole interactions.
• Employs a hybrid functional extrapolation scheme to reduce computational cost while maintaining accuracy for large systems.
• Validated the method on Xe@CNT systems using the PBE functional with 'tight' settings in FHI-aims, with results extrapolated to higher accuracy.

Experimental results

Research questions

• RQ1 What is the magnitude and physical origin of the Coulomb interaction between dipolar quantum fluctuations beyond the dipole approximation in van der Waals systems?
• RQ2 Why do standard dispersion models fail to capture solvent effects on dispersion interactions in proton-bound dimers?
• RQ3 How do long-range Coulomb corrections influence the ultrafast flow of water through carbon nanotubes versus boron nitride nanotubes?
• RQ4 Can higher-order multipole contributions explain the ordering of large polarizable molecules on metal surfaces?
• RQ5 To what extent do these corrections alter the qualitative nature of long-range van der Waals interactions in confined nanostructures?

Key findings

• The Coulomb Singles (CS) contribution to van der Waals interactions can reach up to 6 kJ/mol in supramolecular systems and nanoconfined materials, challenging the assumption that such effects are negligible.
• The CS term arises from two components: a classical electrostatic energy change due to polarization and a correlation correction to dipole-dipole interactions, both of which are non-trivial and non-additive.
• The CS interaction vanishes in isotropic 3D vacuum due to symmetry but becomes significant under geometric confinement, such as in nanotubes or layered materials.
• The inclusion of CS resolves discrepancies in modeling solvent effects on dispersion interactions in proton-bound dimers, where standard dipole-based models fail.
• The CS correction qualitatively alters the long-range behavior of vdW interactions in nanostructures, explaining phenomena like ultrafast water transport in carbon nanotubes and molecular ordering on metal surfaces.
• The method provides a practical, many-body approach to include beyond-dipolar effects in large molecular systems, enabling accurate modeling of collective behavior in advanced materials.