Introduction

To describe the interaction of a molecule with an external field, the concept of molecular response comes into play. Formally, it refers to response theory, namely the calculation of field-induced molecular properties through changes in expectation values [1]. The molecular response can take a variety of forms depending on the perturbing field, which can be electric, magnetic, or both, static or dynamic as in the case of the electromagnetic radiation. For example, in the presence of an electric field (time-independent or time-dependent), the electron cloud is polarized and an induced electric moment (static or dynamic) appears as a consequence of the perturbation. In the case of a magnetic field, electron currents start flowing within the electron cloud, thus providing an induced magnetic moment. Such induced electric and magnetic moments can be measured and may serve in technological applications.