Convergence of Computed Aqueous Absorption Spectra with Explicit Quantum Mechanical Solvent

Area of Science:

• Computational Chemistry
• Spectroscopy
• Molecular Modeling

Background:

• Accurate condensed phase simulations necessitate models capturing both short- and long-range interactions, crucial in aqueous solutions due to hydrogen bonding and polarization.
• Quantum mechanical (QM) treatments can model short-range solute-solvent interactions, but the extent of QM solvent needed for accuracy remains unclear.

Purpose of the Study:

• Investigate the impact of explicit QM solvent on computed absorption spectra for solutes of varying polarity.
• Determine the necessary size of the QM region for converged absorption spectra calculations.
• Optimize computational efficiency by exploring mixed basis sets.

Main Methods:

• Calculated absorption spectra for solutes using QM/MM (Quantum Mechanics/Molecular Mechanics) methods, varying the QM region size up to 400 water molecules.
• Employed time-dependent density functional theory (TD-DFT) with a range-separated hybrid functional and configuration interaction singles (CIS).
• Tested a mixed basis set approach, varying basis function density based on proximity to the chromophore.

Main Results:

• Achieved convergence of absorption spectra by adjusting the QM region size.
• Convergence rate correlated with solute polarity for CIS but not TD-DFT.
• Larger basis sets accelerated spectral convergence.
• A mixed basis set approach effectively reduced computational cost while maintaining accuracy.

Conclusions:

• The size of the QM region and basis set choice significantly impact the accuracy and efficiency of condensed phase simulations.
• Mixed basis sets offer a promising strategy for cost-effective, accurate spectral calculations.
• Understanding these factors is key for reliable molecular simulations.