### What is ATLAS?

**What is ATLAS?**

The ATLAS (Ab initio orbiTaL-free density functionAl theory Software) is an orbital-free DFT based efﬁcient linear scaling ﬁrst-principle electronic structure calculation method and its same-name computer software. This approach can be used to large scale (e.g. system contains millions atoms) real materials (e.g. , alloys, nanoparticles) simulations while still accounting for the totality of the valence electrons.

The package is protected by the Copyright Protection Center of China with the registration No. xxx and classiﬁcation No. xxxx.

**What can ATLAS do?**

1. Electron density optimization for given materials (geometry) structure.

2. Calculation of force and stress.

3. Structure optimization.

4. Molecular dynamics simulation (support microcanonical (NVE), canonical (NVT), and isothermalisobaric (NPT) ensembles.)

5. Support both periodic systems and isolated systems with all advanced nonlocal kinetic energy functionals (KEDFs) and local pseudo-potentials(LPPs)

**Major Techniques Employed **

1. Real space high-order ﬁnite-difference (FD) method for general grid (both orthorhombic non-orthorhombic cell) is adopted for Laplace operator and Poisson equation solver. Real-space FD expansion has three obvious advantages:

A, Parallel algorithms are easily achieved, enabling high-speed calculation using a massively parallel computer.

B, There is no barrier to switching between periodic and non-periodic system.

C, Since arbitrary boundary conditions are available, the method can treat a model that corresponds to the actual experiment.

2. Direct minimization of the total energy approach is adopted, in electron density optimization, to ensure the numerical stability.

3. Plane waves method is used to evaluate the convolutions in nonlocal KEDFs to improve the computational efﬁciency.

4. Pencil (2D) decomposition scheme is adopted in parallelizing FFTs to take full advantage of the massive parallelism available on modern high performance computing architectures for large-scale simulations.

5. Proposed new efﬁcient linear-scaling approach to calculate the ion-electron potential, which can enable large-system (beyond millions atoms) simulation.

6. Advanced nonlocal KEDFs (MGP, XWM, LX(X=WT, MGP0, MGP), LDAK-X(X=WT, MGP, MGP0, SM, Perrot) are ﬁrstly adopted and benchmarked in ATLAS.