Abstract <p>In this paper, we study the influence of the quantum effect on the optical characteristics of a nanoparticle dimer with a nanometer gap. Structures with nanogaps are used for deciphering spectra of individual molecules and developing various types of biosensors in the devices designed for the express analysis of the virus-containing environment. We chose gold and sodium as the particle materials. The quantum effect is taken into account on the basis of mesoscopic boundary conditions, incorporating the Feibelman parameters. The simulation was performed by means of the Discrete Source Method, which provides the analysis of optical characteristics of the nanostructures with the estimation of accuracy of the obtained result. The Feibelman parameter allowed us to consider the effects of ‘‘spill in’’ of induced electrons in gold particles and the ‘‘spill out’’ of the electrons from the metal in sodium particles. As a result, significant differences in the optical characteristics of these materials were discovered, especially when the field intensity was considered in the nanometric gap between the particles. It was found that when the quantum effect is taken into account, the field intensity in the gap between the sodium particles can be an order of magnitude greater than in the classical case. In addition, a shift of the plasmon resonance to the long-wave region was observed, reaching a value of 30 nm.</p>