Stress calculation in linear scaling DFT: convergence and dynamics

We present the approach needed to calculate stress within density functional theory (DFT) using a localised orbital basis, both for exact diagonalisation and linear scaling approaches, and demonstrate our implementation within the large scale DFT code Conquest. For the linear scaling approach, we test the rate of convergence of stress with density matrix range, and compare it to the convergence of energy and forces for different materials with a range of band gaps. We show that excellent convergence is found for modest cutoffs (below 0.1GPa error for range of 20a0), and show that large-scale isothermal-isobaric molecular dynamics is stable and accurate, showing negligible drift over picoseconds of dynamics.