Going beyond the frozen core approximation: development of coordinate-dependent pseudopotentials and application to...

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Molecular Modeling

Background:

• Mixed quantum/classical (MQC) simulations use pseudopotentials to couple quantum and classical treatments.
• Current pseudopotentials often rely on the frozen core approximation (FCA), ignoring core orbital changes with nuclear coordinates.
• FCA can lead to inaccuracies in valence electron wave function calculations.

Purpose of the Study:

• To develop coordinate-dependent pseudopotentials that overcome the limitations of the frozen core approximation (FCA).
• To account for polarization effects on atomic cores in different chemical environments.
• To improve the accuracy of MQC simulations by including explicit dependence of pseudopotentials on nuclear coordinates.

Main Methods:

• Derived analytic expressions for pseudopotentials as explicit functions of nuclear coordinates.
• Developed a coordinate-dependent pseudopotential for the bonding electron of the sodium dimer cation (Na2+).
• Performed one-electron MQC simulations using the new pseudopotential and compared results to FCA and full Hartree-Fock (HF) calculations.

Main Results:

• The coordinate-dependent pseudopotential accurately reproduces HF calculation results across all internuclear separations.
• MQC simulations with the new potential showed significant accuracy improvements over FCA for Na2+.
• FCA calculations resulted in overbinding and ~10% errors in bond length and vibrational frequency, which were corrected by the new potential.

Conclusions:

• Coordinate-dependent pseudopotentials offer enhanced accuracy in MQC simulations without increased computational cost.
• The study highlights the importance of accounting for core orbital changes, even in simple electronic systems like Na2+.
• This approach provides a pathway to more reliable molecular simulations by addressing limitations of the frozen core approximation.