We study geometric and energetic factors that partake in modifying properties of polymeric melts via inserting well-dispersed nanoscopic particles (NP). Model systems are cis-1,4-polybutadiene melts including a single atomic clusters of size varied in the range 10150 atoms (37 angstrom in radius; 0.11.5% v/v). We modify the interactions between the chains and the particle by tuning attractive van der Waals interactions. Using molecular dynamics, we study equilibrium fluctuations and dynamical properties at the interface. The NPs move in the polymer matrix in two different regimes corresponding to trapped and free diffusion, depending on the NP size. Furthermore, degree of crowding around the NP by the polymer chains is quantified. Effect of NP size and interaction strength both on volume and volumetric fluctuations is manifested in mechanical properties, quantified here by bulk modulus, K. Tuning NP size and nonbonded interactions results in similar to 15% enhancement in K by addition of a maximum of 1.5% v/v NP. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012