Ionization energies in solution with the QM:QM approach.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Physical Chemistry

Background:

• Calculating electronic processes in condensed phases is computationally demanding.
• Existing methods like QM/MM or full DFT have limitations.
• Accurate simulation of condensed-phase energetics is crucial for understanding chemical phenomena.

Purpose of the Study:

• To present a practical fragment-based quantum mechanics:quantum mechanics (QM:QM) scheme.
• To evaluate the QM:QM method for simulating vertical electronic processes in condensed phases.
• To compare the QM:QM approach with existing methods for accuracy and suitability.

Main Methods:

• Decomposition of large molecular systems into small, electrostatically interacting fragments.
• Self-consistent field (SCF) calculations for fragment energies within the generated field.
• Summation of fragment energies to obtain the total system energy.
• Application to cytosine and a sodium cation to simulate vertical ionization energies (VIEs).

Main Results:

• The QM:QM scheme accurately simulates the shift in vertical ionization energies (VIEs) from gas to bulk phase.
• DFT-level predictions for solvent shifts and peak widths show good agreement with experimental data for cytosine and sodium cation.
• The QM:QM approach demonstrates superior suitability compared to QM/MM and full DFT methods.

Conclusions:

• The fragment-based QM:QM method offers a practical and accurate approach for condensed-phase electronic energetics.
• The method shows promise for simulating other electronic processes like Auger decay.
• QM:QM provides a viable alternative to computationally expensive or less accurate existing methods.