The FHI-aims code

An Introduction to the FHI-aims Code

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Who we are

Amazing Community of Contributors

Beginning of the Code: Volker Blum, Karsten Reuter, Matthias Scheffler at Fritz Haber Institute, 2004
Since then: Hundreds of individuals, without whom FHI-aims would not exist
Direct contributors to FHI-aims: ~150

How we are organized

FHI-aims is licensed through MS1P e.V. - a registered nonprofit supporting basic science
User registration and code download at fhi-aims.org
Primary communication platform: Slack Workspace
Code developement at the FHI-aims GitLab including stable versions, issue tracker, wiki, and build instructions

Why FHI-aims?

Vision:

Quantum mechanics based simulations of molecules, materials and their properties without a priori precision and accuracy limitations

Why FHI-aims? – Critical choice

Numeric atom-centered basis functions

for an accurate representation of occupied orbitals and densities

Algorithmic choices and priorities

All-electron, full potential
Non-periodic and periodic systems on equal footing
Scalability to large systems (thousands of atoms) without precision limitations
Seamless scalability from laptop to massively parallel HPC architectures
Density functional theory and correlated methods (RPA, GW, ...)

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Numeric Atom-Centered Basis Functions

\[\begin{aligned} \varphi_{i[lm]} (r) & = \frac{u_i(r)}{r} Y_{lm} (\Omega) \end{aligned} \]

$u_i(r)$: Localized radial function. Flexible choice - “Anything you like.”

Solution to the free atom radial Schrödinger equation:

\[\begin{aligned} \left[ - \frac{1}{2} \frac{\text{d}^2}{\text{d}r^2} + \frac{l(l+1)}{r^2} + v_i(r) +v_\text{cut}(r) \right] u_i(r) = \epsilon_i u_i(r) \end{aligned} \]

free-atom like: $v_i(r) = v^\text{DFT}_\text{free atom}(r)$
hydrogen-like: $v_i(r) = \frac{Z_i}{r}$
free ions, harmonic oscillators (Gaussians), ...

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Our basis set library

List of NAO basis functions for all elements (1-102)
From fast qualitative to meV-converged total energies (for LDA/GGA/hybrid density functionals)

How to construct them? All details in the paper

V. Blum, R. Gehrke, F. Hanke, P. Havu,V. Havu, X. Ren, K. Reuter and M. Scheffler, “Ab Initio Molecular Simulations with Numeric Atom-Centered Orbitals”, Computer Physics Communications 180, 2175-2196 (2009)