Recorded 29 March 2023. David Bowler of University College London presents "Large-scale and linear scaling DFT: why we need it, and how we do it" at IPAM's Increasing the Length, Time, and Accuracy of Materials Modeling Using Exascale Computing workshop. Abstract: We will survey the underlying theory behind the large-scale and linear scaling DFT code, CONQUEST[1], which shows exceptional parallel scaling (demonstrated up to 200,000 cores) and can be applied to up to ten thousand atoms with diagonalisation, and millions of atoms with linear scaling. We will give details of the representation of the density matrix and the approach to finding the ground state, and discuss the implementation of molecular dynamics with linear scaling. We will give an overview of the performance of the code, and provide examples of recent developments. We will also discuss the recent application of CONQUEST to complex ferroelectric systems with up to 5,000 atoms[2,3]. We studied the local polarisation textures[2] of PbTiO3 thin films on SrTiO3. We observed the formation of polar vortices in a thick film (9 layers), while thinner films (3 layers) cannot support these, instead showing a polar wave with chiral bubbles forming at the surface; we have extended these studies using linear scaling to investigate the interaction of domain walls with surface trenches[3]. [1] A. Nakata et al., J. Chem. Phys. 152, 164112 (2020) [2] J. S. Baker and D. R. Bowler, Adv. Theory Simul. 3, 2000154 (2020) [3] J. S. Baker and D. R. Bowler, Phys. Rev. Lett. 127, 247601 (2021) Learn more online at: http://www.ipam.ucla.edu/programs/workshops/workshop-i-increasing-the-length-time-and-accuracy-of-materials-modeling-using-exascale-computing/