Intermolecular Interactions in Static Electric Fields Studied with Quantum Mechanics and Quantum Electrodynamics

In the present work, the interactions between neutral molecular systems subject to external static electric fields are studied. To comprehensively explore the effects of external fields on intermolecular interactions, the two most reliable frameworks in the subject, namely molecular quantum mechanics and quantum electrodynamics are employed while atomic and molecular responses are modeled using quantum Drude oscillators (QDO). In the first part of the work, the focus is to understand the interplay between dispersion and field-induced forces in two-body systems for both nonretarded and retarded ranges of inter-species distances. To identify the origin and the mechanism responsible for different field-induced interactions, a complementary approach based on classical electrodynamics with a zero-point radiation field, namely stochastic electrodynamics, is employed. The results show that neglecting higher-order contributions coming from field-induced hyperpolarizabilities of atoms, the dispersion interaction remains unchanged by the external uniform static field, for both regimes. However, using an external static field one can control the magnitude and characteristics of intermolecular interactions. The second part of the work is devoted to the extension of the study to many-body interacting systems. There, the total interaction energy in systems with many interacting atoms or molecules is obtained by extending the well-established theory of many-body dispersion (MBD) interactions to the presence of external static electric fields. Diagonalization of the Hamiltonian of the system in the nonretarded regime and in the framework of quantum mechanics yields the total energy of the interacting system in terms of the corresponding normal mode frequencies. Subtraction of the energy of the non-interacting QDOs-in-fields from the total energy of the interacting system results in the many-body interaction energy. The impact of the field-induced many-body contributions is investigated for a benzene dimer as well as for two carbyne chains. Varying the number of carbon atoms per chain demonstrates the significance of the field-induced many-body terms in the interplay between dispersion and field-induced interactions. Such contributions can be of great importance for controlling delamination and self-assembly of materials in static electric fields.