where b ( X ) is the drift field and B e the diffusion matrix. The effective diffusion tensor can vary in space D ( X ) = 1 2 B ( X ) B T X T ( X T denotes the transpose of X ). This equation is not derived but assumed. However the diffusion coefficient should be smooth enough as any discontinuity in D should be resolved by a spatial scaling to analyse the source of discontinuity (usually inert obstacles or transitions between two medias). The observed effective diffusion tensor is not necessarily isotropic and can be state-dependent, whereas the friction coefficient γ γ remains constant as long as the medium stays the same and the microscopic diffusion coefficient (or tensor) could remain isotropic.