What is the maximum system size ASAP can handle?

The number of atoms you can simulate depends on the engine you choose. ASAP connects to two DFT codes that use different methods to calculate electronic structures:

1. SIESTA (LCAO Approach)
SIESTA uses a Linear Combination of Atomic Orbitals (LCAO) basis set.
Strengths: This approach is highly efficient and features linear scaling of computational cost with respect to the number of atoms.
System Size: Because of the localized nature of the basis set, it is the ideal choice for large-scale systems containing several hundred or even thousands of atoms.
Use Case: Ideal for large biological molecules, nanostructures, and complex interfaces where the scale of the system makes other methods prohibitively slow.

2. Quantum ESPRESSO (Plane Wave Approach)
Quantum ESPRESSO (QE) utilizes a Plane Wave (PW) basis set.
Strengths: Plane waves are spatially uniform and provide a systematic way to reach the “complete basis set” limit, making them highly reliable for high-precision calculations.
Limitations: The computational cost of PW methods typically scales more steeply (O(N$^3$)) than LCAO. This generally limits the practical system size to a few hundred atoms, depending on the available HPC resources.
Use Case: Best suited for high-precision bulk properties, complex magnetic states, or systems where absolute convergence is the primary goal.