Microscopic elasticity from MD part I: Bulk solid and fluid systems

Computational modeling such as molecular dynamics (MD) and Monte Carlo simulations can be used to estimate the elastic properties of materials through various stress and strain relationships. Here, we demonstrate the effectiveness of the stress-stress fluctuation (SSF) method to estimate the elastic properties of simple van der Waals and molecular materials. The SSF method allows computation of the complete elasticity tensor from a single equilibrium simulation without requiring any type of deformation. While extensively used to characterize the elastic coefficients of crystalline solid and glassy systems, application of the SSF method to fluid systems and biomaterials has been limited. Starting with argon in the solid, liquid, and gas phases, we show that the SSF method gives elastic coefficients and moduli in excellent agreement with values obtained with the explicit deformation and volume fluctuation methods. Comparison of the elastic coefficients and bulk modulus for solid argon with previous computational studies and experimental data provide further validation of our numerical implementation. Beyond argon, we show that the elastic properties of molecular fluids simulated with the CG MARTINI force-field, which include multi-body interactions such as angle potentials, are also accurately captured by the SSF method. Moreover, the impulsive correction for truncated potentials is essential to obtain accurate values for these fluids and vanishing shear moduli. Our results highlight the broad applicability of the SSF method across a broad range of systems and lays the foundation for its use to characterize the elastic properties of complex molecular systems.