Developing a Molecular Theory of Electromechanical Responses

Developing a bottom-up (molecular) theory for the electromechanical response of aperiodic materials is a prerequisite for understanding the piezoelectric properties of systems such as nanoparticles, (non-crystalline) polymers, or biomolecule agglomerates. The focus of this publication is to establish a new language and formalism for describing molecular piezoelectric responses. More specifically, we define the molecular piezoelectric response tensor d, which necessarily differs from the known bulk definition due to the anisotropy and inhomogeneity at the molecular scale, and derive an analytical theory to calculate this tensor. Based on this new theory, we develop a computational procedure for practical calculations of piezoelectric matrices for molecular systems. Our studies demonstrate that the new analytical theory yields results that are consistent with fully numerical computations. This publication is the first in a series; this work establishes the theoretical molecular foundation and follow-up publications will show how to bridge molecular and macroscopic piezoelectric responses. It is expected that the present work will aid in developing design strategies for piezoelectric materials by revealing connections between molecular structure and piezoelectric response. We expect that the language and formalism developed here may also be useful to describe mechanochemical phenomena.