Multicomponent Orbital-Optimized Perturbation Theory with Density Fitting: Anharmonic Zero-Point Energies in...

Area of Science:

• Quantum chemistry
• Computational chemistry
• Chemical physics

Background:

• Nuclear quantum effects, like zero-point energy, significantly influence chemical and biological processes.
• The nuclear-electronic orbital (NEO) framework quantizes both electrons and nuclei simultaneously.
• Accurate theoretical methods are needed to model these quantum effects.

Purpose of the Study:

• To implement and apply an efficient density-fitted NEO scaled-opposite-spin orbital-optimized second-order Møller-Plesset perturbation theory (NEO-SOS'-OOMP2) method.
• To assess the accuracy of NEO-SOS'-OOMP2 for calculating proton affinities and relative energies.
• To investigate the role of anharmonicity in protonated water clusters.

Main Methods:

• Implementation of density-fitted NEO-SOS'-OOMP2.
• Calculation of proton affinities for small molecules.
• Computation of relative energies for protonated water tetramer, hexamer, and heptamer isomers.
• Comparison with experimental data and previous theoretical calculations (e.g., NEO coupled cluster).

Main Results:

• The NEO-SOS'-OOMP2 method accurately predicts proton affinities, matching experimental precision.
• Relative energies of protonated water tetramer isomers calculated by NEO-SOS'-OOMP2 agree with prior NEO coupled cluster results.
• Anharmonicity was found to be critical for accurate relative energy calculations in larger protonated water clusters (hexamers and heptamers).

Conclusions:

• The density-fitted NEO-SOS'-OOMP2 method provides an efficient and accurate approach for including nuclear quantum effects.
• This method successfully captures anharmonic zero-point energies, making it valuable for studying reaction pathways and dynamics.
• The findings highlight the importance of quantum mechanical treatment of nuclei in chemical systems.