The most common method to calculate the surface stresses is by calculating the surface free energy and its derivative with respect to elastic strain. Different methods have been used such as first principles, atomistic potential calculations and molecular dynamics simulations, with density functional theory most common. A large tabulation of calculated values for metals has been given by Lee et al. Typical values of the surface energies are 1-2 Joule per metre squared ( J m − − 2 ), with the trace of the surface stress tensor g i j in the range of -1 to 1 J m − − 2 . Some metals such as aluminum are calculated to have fairly high, positive values (e.g. 0.82) indicating a strong propensity to contract, whereas others such as calcium are quite negative at -1.25, and others are close to zero such as cesium (-0.02).