Modeling positrons in molecular electronic structure calculations with the nuclear-electronic orbital method

Area of Science:

• Quantum Chemistry
• Computational Physics
• Positron Science

Background:

• The nuclear-electronic orbital (NEO) method traditionally studies interactions involving nuclei and electrons.
• Extending computational methods to include positrons is crucial for understanding exotic matter and chemical systems involving antimatter.

Purpose of the Study:

• To modify and extend the NEO method to accommodate positrons, creating a framework for mixed positronic-electronic wavefunctions.
• To develop and implement computational strategies for calculating electron-positron annihilation rates within the NEO framework.
• To apply these advanced computational tools to model the positronium hydride (PsH) and e+LiH systems.

Main Methods:

• Modification of the NEO method to include the mass of the positron instead of the proton.
• Implementation of NEO-Hartree-Fock (NEO-HF), NEO-second-order perturbation theory (NEO-MP2), and NEO-full configuration interaction (NEO-FCI) for positronic systems.
• Optimization of positronic and electronic basis sets for accurate energy and annihilation rate calculations.

Main Results:

• Successful application of the modified NEO method to positronic systems, including PsH and e+LiH.
• Calculation of mixed positronic-electronic wavefunctions, energies, and electron-positron annihilation rates.
• Investigation of basis set effects on correlation energies and annihilation rates, and computation of equilibrium properties for e+LiH.

Conclusions:

• The extended NEO method provides a robust framework for studying systems with both electrons and positrons.
• The developed methods accurately compute molecular properties and annihilation rates for positronic systems.
• This work opens new avenues for theoretical investigations in positron chemistry and physics.