Self-interaction error overbinds water clusters but cancels in structural energy differences.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Materials science

Background:

• Density Functional Approximations (DFAs) are widely used for electronic structure calculations.
• Self-interaction error (SIE) is a known limitation in many DFAs, affecting accuracy.
• Water clusters are important systems for studying intermolecular interactions and hydrogen bonding.

Purpose of the Study:

• To quantify the impact of self-interaction errors on the binding energies of water clusters.
• To assess the effectiveness of the Fermi-Löwdin orbital self-interaction correction (FLOSIC) method for improving DFA accuracy.
• To investigate the nature of errors in different DFA families for water clusters.

Main Methods:

• Calculated binding energies of water clusters (up to 8 molecules) using three representative DFAs: LDA, PBE, and SCAN.
• Applied the Fermi-Löwdin orbital self-interaction correction (FLOSIC) method to correct for SIE in the DFAs.
• Decomposed total binding energies into many-body components (one-body, two-body, etc.).

Main Results:

• Overbinding in water clusters using standard DFAs is not solely a density-driven error.
• FLOSIC improves binding energy accuracy towards higher-level reference values, without altering energetic ordering of isomers.
• Self-interaction corrected SCAN significantly reduces mean error in water hexamer binding energies (from ~42 to <14 meV/molecule).
• Large two-body interaction errors in SCAN are substantially reduced by SIE correction; higher-order errors remain small.

Conclusions:

• Orbital-by-orbital removal of self-interaction is crucial for accurate descriptions of water clusters.
• Combining FLOSIC with appropriate DFAs like SCAN offers a promising route to improved theoretical models for hydrogen-bonded systems.
• The study highlights the importance of addressing SIE for reliable predictions of cluster properties.