The electronic structure of liquid water within density-functional theory

Area of Science:

• Computational physics and chemistry
• Quantum mechanics and condensed matter physics

Background:

• Previous computational studies of liquid water focused primarily on ground-state properties.
• Interpreting recent spectroscopic measurements of water necessitates accurate theoretical models of excited electronic states.

Purpose of the Study:

• To investigate the electronic properties of liquid water at ambient conditions.
• To determine the impact of supercell size and k-point sampling on the accuracy of electronic structure calculations, particularly for unoccupied electronic density of states (u-EDOS).

Main Methods:

• Utilized ab initio density-functional theory within the generalized gradient approximation (DFT/GGA).
• Generated long classical trajectories for large supercells (up to 256 molecules).
• Extracted uncorrelated configurations for electronic structure calculations.

Main Results:

• The density of occupied states is accurately described using 32-molecule supercells with single k-point (k=0) approximation.
• The unoccupied electronic density of states (u-EDOS) is sensitive to finite size effects, with small supercells showing spurious states.
• Accurate u-EDOS can be achieved with smaller supercells by increasing k-point sampling, indicating robustness against long-range structural disorder.

Conclusions:

• Efficiently increasing the accuracy of spectral calculations for water and other molecular liquids is possible.
• The findings provide guidance for optimizing computational resources in electronic structure studies of molecular liquids.