For stress and strain the same optimiser is employed by adding two dummy atoms to the geometry. Their six cartesian coordinates and gradients correspond to the six independent strain and stress components used for the cell optimisation. Stresses are calculated according to Equation (12) and scaled up by an appropriate factor, representing the ratio of default gradient and stress thresholds in atomic units. This allows for seamless integration into the Chemshell structure. Just as in the case of cartesian coordinates, the optimiser provides an update to the strain components. The application of this step is then performed internally according to Equation (14). This way, the procedure can be used in combination with any general optimiser. A charge SCF cycle is performed for the new structure, repeating the protocol. At no point the environment is kept frozen. All molecules have the same internal geometry as expected in a perfect Calculate QM charges q(QM) Charges q(acc) Geometry r(acc) i++