Separation of electron-electron and electron-proton correlation in multicomponent orbital-optimized perturbation...

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Theoretical Chemistry

Background:

• The multicomponent orbital-optimized second-order Møller-Plesset perturbation theory (OOMP2) method accurately calculates protonic properties and nuclear quantum effects.
• Existing methods face computational expense, motivating the development of more efficient approximations.
• Accurate calculation of protonic densities, affinities, and geometries is crucial for understanding multicomponent systems.

Purpose of the Study:

• To introduce and evaluate two approximations of the multicomponent OOMP2 method.
• To demonstrate that electron-proton correlation is sufficient for accurate protonic properties in orbital-optimized methods.
• To reduce the computational cost of the multicomponent OOMP2 method.

Main Methods:

• Developed two approximations of the multicomponent OOMP2 method, focusing on electron-proton correlation during orbital optimization.
• The first approximation simplifies the wave function to one-electron one-proton excitations.
• The second approximation further neglects specific terms in orbital-rotation gradients, reducing computational scaling to N_e³ for orbital optimization.

Main Results:

• Both approximations yield accurate protonic densities, protonic affinities, and optimized geometries, comparable or superior to the original OOMP2 method.
• The second approximation significantly reduces computational scaling for orbital optimization iterations.
• Despite simplifications, the accuracy of key chemical properties is maintained or enhanced.

Conclusions:

• Orbital optimization with only electron-proton correlation is sufficient for accurate protonic properties in multicomponent systems.
• The developed approximations offer a computationally efficient alternative to the original multicomponent OOMP2 method.
• These findings pave the way for more accessible and efficient theoretical studies of systems with significant nuclear quantum effects.