Nuclear-electronic orbital second-order coupled cluster for excited states

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The nuclear-electronic orbital (NEO) framework treats nuclei quantum mechanically alongside electrons.
• Existing excited-state NEO methods are computationally expensive or limited in accuracy.
• Accurate modeling of vibronic transitions requires methods capturing electron-proton correlation.

Purpose of the Study:

• Introduce and benchmark excited-state nuclear-electronic orbital coupled cluster with approximate second-order doubles (NEO-CC2) and its scaled-opposite-spin variant (NEO-SOS'-CC2).
• Evaluate the methods' ability to describe electronic, vibrational, and mixed electron-proton excitations.
• Assess the impact of electron-proton correlation scaling on accuracy.

Main Methods:

• Development of excited-state NEO-CC2 and NEO-SOS'-CC2 methods.
• Benchmarking on positronium hydride with quantum mechanical electrons and positron.
• Testing on triatomic molecules with a quantum proton.

Main Results:

• NEO-CC2 shows qualitative agreement with exact results for positronium hydride.
• NEO-SOS'-CC2 achieves near-quantitative accuracy for positronium hydride with increased electron-positron correlation.
• Both methods capture vibrational features like overtones and combination bands, and mixed electron-proton double excitations in molecular systems.
• Electron-proton correlation scaling improves accuracy across tested systems.

Conclusions:

• NEO-SOS'-CC2 offers a computationally feasible approach for describing single and mixed protonic and electronic excitations.
• The method approaches the accuracy of more computationally demanding techniques.
• Further improvements are needed to address basis set effects and fully capture excited-state electron-proton correlation for quantitative accuracy.