Calculating excitonic interactions using transition currents with application to PTCDA.

Area of Science:

• * Molecular interactions and electronic structure theory.
• * Computational chemistry and condensed matter physics.

Background:

• * Molecular interactions are typically modeled using Coulomb interaction and charge densities.
• * Electronic wavefunctions of molecules in assemblies often do not overlap.

Purpose of the Study:

• * To compare interaction calculations using transition charge densities versus transition current densities.
• * To investigate the impact of electronic-structure theory approximations on these calculations.
• * To analyze molecular interactions for 3,4,9,10-perylenetetracarboxylicacid-dianhydride (PTCDA) on surfaces.

Main Methods:

• * Calculation of matrix elements using both charge and current densities.
• * Employing electronic-structure theory for molecular wavefunctions.
• * Performing calculations for PTCDA molecules in various arrangements, including on KCl and NaCl surfaces.

Main Results:

• * Exact agreement between charge and current density methods for exact wavefunctions.
• * Marked differences observed when using wavefunctions from electronic-structure theory.
• * Discrepancies attributed to transition energy errors and wavefunction quality, especially at small molecular separations.
• * Comparison with the point-dipole approximation for PTCDA on surfaces.

Conclusions:

• * The choice of density (charge vs. current) significantly impacts interaction calculations when using approximate wavefunctions.
• * Wavefunction quality is crucial for accurate interaction energy predictions.
• * A novel algorithm is presented for efficient integral calculations in molecular interactions.