The CONQUEST code: large scale and linear scaling DFT

TL;DR

CONQUEST is a highly scalable DFT code capable of handling systems from hundreds to over a million atoms by employing local basis sets, sparse matrices, and both exact and linear scaling solvers, optimized for massively parallel platforms.

Contribution

It introduces a DFT code that combines exact and linear scaling methods with local basis sets and sparse matrix operations for extremely large-scale calculations.

Findings

Supports systems up to 1,000 atoms with modest resources

Enables calculations of over 1,000,000 atoms on thousands of cores

Demonstrates nearly perfect weak scaling on supercomputers

Abstract

CONQUEST is a DFT code which was designed from the beginning to enable extremely large-scale calculations on massively parallel platforms, implementing both exact and linear scaling solvers for the ground state. It uses local basis sets (both pseudo-atomic orbitals, PAOs, and systematically convergent B-splines) and sparse matrix storage and operations to ensure locality in all aspects of the calculation. Using exact diagonalisation approaches and a full PAO basis set, systems of up to 1,000 atoms can be modelled with relatively modest resources (200-500 cores), while use of multi-site support functions (MSSF) enable calculations of up to 10,000 atoms with similar resources. With linear scaling, the code demonstrates essentially perfect weak scaling (fixed atoms per process), and has been applied to over 1,000,000 atoms, scaling to nearly 200,000 cores; it has been run on both the K computer and Fugaku, among other computers. CONQUEST calculates the total energy, forces and stresses exactly, and allows structural optimisation of both ions and simulation cell. Molecular dynamics calculations within the NVE, NVT and NPT ensembles are possible with both exact diagonalisation and linear scaling[6]. The code interfaces with LibXC to implement LDA and GGA functionals, with metaGGA and hybrid functionals under development. Dispersion interactions can be included using semi-empirical methods (DFT-D2/3, TS) and vdW-DF. The polarisation can be calculated using Resta's approach.