s. The preliminary shear stress is introduced by particle pair to avoid spurious stress and deformation. Note that, for the shear stress term, due to the mismatch of the divergence contribution between the pair-wise second-order derivative Laplacian formulation and particle-wise first-order gradient formulation, a correction factor slight large than one is introduced. Note that, if you see time step size goes unusually small, it may be due to the determinate of deformation matrix become negative. In this ca