PAW for OpenAtom

In order to treat complex and large systems like metal-semiconductor heterostructures and DNA molecules, we developed an ab-initio molecular dynamics software OpenAtom, which is highly scalable on multi-petaflops parallel platforms. The plane-wave based Kohn-Sham Density Functional Theory (KS-DFT) computations in OpenAtom are currently performed using norm conserving non-local pseudopotentials, yielding a highly parallelize fast method. However, for many systems of interest, the Projector Augmented Wave (PAW) approach of Bl"{o}chl has several important advantages, most notably (1) less demanding plane-wave cutoff energy due to the splitting of the KS states into delocalized and core parts and (2) direct access to the core-states if needed for NMR and other applications. However, traditional PAW based KS-DFT scales as $\sim N^3$ and the parallel performance is poor compared to the plane-wave approach. To solve this problem, we have developed a PAW based KS-DFT with $\sim N^2 \mathrm{log} N$ scaling enabled by Euler Exponential Spline (EES) and FFT's to form a 4 grid multi-resolution method . We have also increased the accuracy of the long-range interactions by applying Ewald summation methods to the Hartree and electrion-ion external energies. Our development, as implemented in OpenAtom, inherits all the merits of the PAW method with significantly higher parallel and scalar efficiency.