Multicomponent Coupled Cluster Singles and Doubles with Density Fitting: Protonated Water Tetramers with Quantized...

Area of Science:

• Quantum chemistry
• Computational chemistry
• Theoretical chemistry

Background:

• Nuclear quantum effects, such as zero-point energy, significantly influence chemical properties.
• The nuclear-electronic orbital (NEO) approach accounts for these effects by treating nuclei and electrons quantum mechanically.
• Accurate computational methods are needed to model these phenomena in larger systems.

Purpose of the Study:

• To implement and assess the nuclear-electronic orbital coupled cluster with singles and doubles (NEO-CCSD) method combined with a density fitting (DF) approximation.
• To enhance computational efficiency for calculations involving multiple quantum protons.
• To evaluate the performance of the NEO-DF-CCSD method for chemical accuracy in predicting proton affinities and isomer energies.

Main Methods:

• Implementation of traditional and t1-transformed NEO-CCSD methods.
• Integration of a density fitting (DF) scheme to approximate four-center two-particle integrals.
• Application to proton affinities and protonated water tetramers with quantum mechanical protons.

Main Results:

• The density fitting approximation significantly enhances computational efficiency.
• The NEO-DF-CCSD method achieves chemical accuracy in predicting proton affinities.
• Accurate isomer energy ordering for protonated water tetramers, highlighting the importance of zero-point energy contributions.

Conclusions:

• The NEO-DF-CCSD method is a computationally efficient and accurate approach for incorporating nuclear quantum effects.
• This method is capable of handling larger molecules with multiple quantum protons.
• The study provides a foundation for future advancements in quantum chemistry calculations involving nuclear quantum effects.