An OrthoBoXY-method for various alternative box geometries.

We have shown in a recent contribution [Busch and Paschek, J. Phys. Chem. B , 2023 127 , 7983-7987] that for molecular dynamics (MD) simulations of isotropic fluids based on orthorhombic periodic boundary conditions with "magical" box length ratios of L z / L x = L z / L y = 2.7933596497, the computed self-diffusion coefficients D x and D y in x - and y -direction become system size independent. They thus represent the true self-diffusion coefficient D 0 = ( D x + D y )/2, while the shear viscosity can be determined from diffusion coefficients in x -, y -, and z -direction, using the expression η = k B T ·8.1711245653/[3π L z ( D x + D y - 2 D z )]. Here we present a more generalized version of this "OrthoBoXY"-approach, which can be applied to any orthorhombic MD box of any shape. In particular, we would like to test, how the efficiency is affected by using a shape more akin to the cubic form, albeit with different box length ratios L x / L z ≠ L y / L z and L x < L y < L z . We use NVT and NpT simulations of systems of 1536 TIP4P/2005 water molecules as a benchmark and explore different box geometries to determine the influence of the box shape on the computed statistical uncertainties for D 0 and η . Moreover, another "magical" set of box length ratios is discovered with L y / L z = 0.57804765578 and L x / L z = 0.33413909235, where the self-diffusion coefficient in x -direction becomes system size independent, such that D 0 = D x .