Application of the exact-factorization density-functional perturbation approach to pentacene crystal and monolayer MoS2

TL;DR

This paper introduces the first implementation of EF-DFPT for extended materials, demonstrating how non-adiabatic electron-phonon interactions influence dielectric properties in pentacene and MoS2.

Contribution

It develops and applies an EF-DFPT method to study non-adiabatic effects on dielectric response in extended periodic systems.

Findings

NA effects are negligible in monolayer MoS2.

NA effects cause significant screening in pentacene.

NA electron-phonon interactions modify dielectric properties.

Abstract

Non-adiabatic effects arising from electron-phonon interactions are often neglected within the Born-Oppenheimer (BO) approximation, which assumes that electronic states adjust instantaneously to nuclear motion. The exact factorization (EF) formalism provides a rigorous framework for treating such effects beyond the adiabatic regime and has recently been adapted to density functional theory (DFT) in the harmonic limit. Building on these foundations, we previously introduced an EF-based perturbative scheme, the EF density-functional perturbation theory (EF-DFPT), that enables the computation of phonon-driven non-adiabatic (NA) corrections to Kohn-Sham (KS) electronic states, up to second order in nuclear displacements. Here, we present the first implementation and application of EF-DFPT to extended periodic materials, focusing on its impact on experimentally relevant observables. Using the pentacene molecular crystal and monolayer MoS2 as representative soft- and stiff-mode systems, respectively, we demonstrate how NA electron-phonon interactions modify the static dielectric response. We show that these modifications originate from the combined effect of NA phonon-dressed electronic wavefunctions and second-order NA energy renormalizations. The resulting behavior is strongly material dependent: NA effects are negligible in monolayer MoS2, whereas in pentacene they lead to pronounced long-range screening effects associated with soft vibrational modes and enhanced electron-phonon coupling.