## 1 Introduction  
Light-induced oscillation of conduction electrons within sub-wavelength regimes in metallic nanoparticles such as Ag and Au is usually termed as localized surface plasmon resonance (LSPR). Owing to their small size, these nanoparticles are able to concentrate light in a sub-wavelength region, resulting in a remarkable enhancement of the electric field in their vicinity [1; 2]. The wave dynamics and electron transport studies in the vicinity of and inside complex nanostructures interconnect different fields, such as metamaterials, plasmonic, quantum transport, and molecular electronics [3; 4]. Properties exhibited by LSPR, e.g., near/far-field enhancement and nanoantenna performance, are tunable by varying certain parameters, such as the particle diameter and the dielectric core and/or shell, and by modifying the surfaces of the nanoparticles [5; 6; 7]. The plasmonic behavior of metal and core-shell nanoparticles has been utilized in many advanced technologies, e.g., imaging, sensing, and biomedical technologies [8]. Most of these applications incorporate the optical response of metallic nanoparticle arrays or clusters of nanoantennas. In order to utilize the optical response of these nanoparticles, detailed studies of the optical properties of the dimers, trimers, and arrays of many particles are extremely vital [9]. One important parameter is the inter-particle distance for regulating the plasmonic response of metallic and core-shell nanoparticle antennas, according to the necessities of already mentioned applications. Near field dipolar coupling is very sensitive to the gap between two plasmonic nanoparticles within the dimer antenna. Therefore, in recent years, the hybridization of electric fields between two nanostructures excited by Electromagnetic (EM) waves and separated by subwavelength regions has become of interest to researchers working in this area. A strong electric field of two or more metal nanoparticles in close proximity has demonstrated an enhancement in biochemical sensors, nanoantenna, and imaging properties [10; 11; 12]. Various types of nano-structures, e.g., nano-rods, bowties, nanowires, nanoparticles, and nano-cubes, have been investigated in attempts to achieve strong electric fields in their closed surroundings [11; 13; 14]. Different theoretical approaches have been attempted so as to explain the interacting electric fields of two nanostructures in close proximity. These theoretical approaches can be divided into two categories: (i) non-local/quantum optical response and (ii) classical/semi-classical optical response [15]. In 2003, it was reported that the wavelength of an inelastically scattered EM wave varies exponentially with the inter-particle gap and is independent of the shape and size of plasmonic nanostructures [16; 17]. Some recent theoretical and experimental studies about nanoparticle dimers have confirmed that exponential behavior, called the \"Ruler equation,\" breaks down when the gap between nanostructures in terms of size becomes smaller [18] since, at such small separation, there is an abrupt increase in near-field coupling of the dimer [19]. In addition to the breakdown of the Ruler equation due to strong near-field coupling, there is an associated transition of electric field distribution which leads to the breeding of two plasmonic modes as the gap between nanoparticles approaches a nanometer or sub-nanometer level [20]. These plasmonic modes strongly affect the major bonding dipolar mode between nanoparticles within the dimer. In recent years, this escalation in near-field coupling and the generation of different dipolar modes have been investigated in various types of pair contacting surfaces, e.g., spherical or flat surfaces of nanowires and face-face or edge-edge nano-cubes [13; 20]. Contrary to previous reports, we suggest in this paper that such breeding plasmonic modes can be explained by a classical modified Drude model, which incorporates size effects in terms of collision frequency.