Bimetallic nanoparticles occupy a unique space in the field of materials science as they display physical and optoelectronic properties that are absent in their monometallic counterparts. These attributes increase their applicability in niche processes where they can perform distinct functions from those achievable with the corresponding pure-element particles. In the current work, we explore the feasibility of performing structural characterization of gold−silver core−shell nanoclusters as well as analyze the transformation between cuboctahedral and icosahedral geometries using a combination of atomistic simulations and electronic structure calculations. We find that size may be a limiting factor in distinguishing between the above two geometries when theoretical vibrational densities of states are employed in characterizing the nanoparticles under consideration. The results from our density functional theory calculations also reveal that the transformation between cuboctahedral and icosahedral geometries occurs via a martensitic, symmetric mechanism for the 147-atom and 309-atom nanoclusters. In addition, the associated transformation barriers for the bimetallic core−shell particles are strongly size-dependent and typically increase with the composition of silver in the nanocluster.