How large are nonadiabatic effects in atomic and diatomic systems?

Area of Science:

• Quantum mechanics
• Computational chemistry
• Atomic and molecular physics

Background:

• Accurate simulation of nonadiabatic systems is crucial for understanding nuclear quantum effects.
• The Born-Oppenheimer approximation is a cornerstone of molecular simulations, but its limitations are increasingly being addressed.

Purpose of the Study:

• To calculate non-relativistic ground-state energies of atomic and molecular systems beyond the Born-Oppenheimer approximation.
• To quantify the impact of nuclear quantum effects on ionization and atomization energies.

Main Methods:

• Utilized the fixed-node diffusion Monte Carlo method.
• Incorporated electron-nuclear coupling by allowing wave function nodes to depend on both electronic and ionic positions.

Main Results:

• Calculated ground-state energies for various atomic and molecular systems.
• Found ionization energies of first-row atoms to be largely independent of the Born-Oppenheimer approximation.
• Observed small but significant effects of nonadiabatic coupling on atomization energies of first-row hydrides.

Conclusions:

• The Born-Oppenheimer approximation is highly accurate for atomic ionization energies.
• Nonadiabatic effects play a measurable role in the energetics of molecular systems, particularly in bond energies.