Calculations of PAH anions: When are diffuse functions necessary?

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are crucial in astrochemistry and materials science.
• Accurate computational modeling of PAH anions is vital for understanding their properties.
• The inclusion of diffuse functions in quantum chemical calculations can impact results.

Purpose of the Study:

• To investigate the impact of diffuse functions on various computational parameters for PAH anions.
• To determine if diffuse functions are necessary for accurate calculations of geometry, total energy, and charge density.
• To assess the necessity of diffuse functions for predicting NMR chemical shifts.

Main Methods:

• Utilized various computational methods to calculate parameters for PAH anions.
• Compared results obtained with and without diffuse functions.
• Analyzed geometry parameters, total energy, charge density, and NMR chemical shifts ((1)H- and (13)C-).

Main Results:

• Excluding diffuse functions had a negligible effect on geometry parameters and total energy calculations.
• Similar negligible effects were observed for charge density, though further data is needed for definitive conclusions.
• The use of diffuse functions was found to be essential for accurate (1)H- and (13)C-NMR shift calculations.

Conclusions:

• Diffuse functions can be omitted for geometry and energy calculations of PAH anions without significant loss of accuracy.
• Charge density calculations may also be performed without diffuse functions, but with caution due to limited data.
• Accurate prediction of (1)H- and (13)C-NMR shifts in PAH anions necessitates the inclusion of diffuse functions in computational models.