Neural Canonical Transformations for Quantum Anharmonic Solids of Lithium

Area of Science:

• Condensed Matter Physics
• Quantum Mechanics
• Materials Science

Background:

• Lithium exhibits complex structural and electronic behavior under pressure, transitioning from a simple cubic structure to more complex phases and undergoing a metal-to-semiconductor transformation.
• Understanding quantum anharmonic effects is crucial for accurately modeling lithium's behavior at finite temperatures and high pressures.
• Previous theoretical and experimental analyses have been complicated by these factors.

Purpose of the Study:

• To investigate quantum anharmonic effects in lithium solids at finite temperatures using a novel computational approach.
• To accurately predict structural transitions and atomic configurations under varying pressures.
• To clarify the stabilization mechanisms of different lithium phases.

Main Methods:

• Employed the neural canonical transformation approach, a variational method utilizing probabilistic generative models.
• Combined a normalizing flow for phonon excited-state wave functions with a probabilistic model for energy level occupation.
• Jointly optimized the model to minimize free energy.

Main Results:

• Quantum anharmonicity was found to lower the body-centered cubic (bcc) to face-centered cubic (fcc) transition temperature compared to classical molecular dynamics.
• Predicted fractional coordinates for lithium atoms in the high-pressure cI16 structure showed good quantitative agreement with experimental data.
• The oC88 structure's stabilization was attributed to the potential energy surface from high-accuracy electronic structure calculations, not thermal or quantum effects.

Conclusions:

• The neural canonical transformation approach provides accurate predictions for quantum anharmonic effects in solids.
• Quantum effects play a significant role in the phase transitions and structural stability of lithium under pressure.
• High-accuracy electronic structure calculations are key to understanding the stabilization of complex phases like oC88 lithium.