Remarks on the validity of the fixed nuclei approximation in quantum electron dynamics

Area of Science:

• Computational Quantum Chemistry
• Theoretical Physics
• Chemical Dynamics

Background:

• The fixed nuclei approximation is commonly used in ab initio correlated electron dynamics simulations.
• This approximation simplifies calculations by treating atomic nuclei as stationary during electron motion.
• The accuracy of this approximation under various conditions requires rigorous testing.

Purpose of the Study:

• To rigorously test the validity of the fixed nuclei approximation in quantum dynamical simulations.
• To introduce and apply the multiconfiguration electron nuclear dynamics (MCEnd) method for this purpose.
• To investigate the impact of the multiconfiguration expansion length on simulation outcomes.

Main Methods:

• Development and implementation of the multiconfiguration electron nuclear dynamics (MCEnd) method.
• Performing quantum dynamical simulations to model electron and nuclear behavior.
• Systematically varying the multiconfiguration expansion length to assess its influence.

Main Results:

• The study provides a quantitative assessment of the fixed nuclei approximation's accuracy.
• MCEnd simulations reveal conditions under which the approximation holds and where it may fail.
• The length of the multiconfiguration expansion significantly affects the simulation results, indicating its importance.

Conclusions:

• The fixed nuclei approximation is not universally valid and its applicability depends on the specific system and dynamics.
• The MCEnd method offers a more accurate and comprehensive approach to studying correlated electron-nuclear dynamics.
• Careful consideration of the multiconfiguration expansion length is crucial for reliable quantum dynamical simulations.