Finite-temperature electronic simulations without the Born-Oppenheimer constraint

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Molecular Dynamics

Background:

• Standard Born-Oppenheimer (BO) molecular dynamics relies on the adiabatic approximation, which fails at higher temperatures.
• Non-adiabatic couplings become significant above the electronic energy gap, affecting molecular properties and chemical reactions.
• Accurate simulation of finite-temperature effects is crucial for understanding many chemical processes.

Purpose of the Study:

• To develop a novel finite-temperature non-adiabatic molecular dynamics method.
• To provide a more accurate description of molecular properties beyond the adiabatic approximation.
• To improve the calculation of free energy and electron entropy at finite temperatures.

Main Methods:

• Introduced a covariant formulation of the electronic partition function for non-adiabatic molecular dynamics.
• Utilized a variational upper bound to the free energy for numerical evaluation.
• Applied the method within a quantum Monte Carlo (QMC) scheme, including electronic correlation (Jastrow) terms.

Main Results:

• The method correctly reduces to the ground-state Born-Oppenheimer (gsBO) limit at zero temperature and the high-temperature limit.
• Improved upper bounds for free energy compared to single BO energy surfaces by efficiently estimating electron entropy.
• Accurately reproduced gsBO behavior at low temperatures for the hydrogen molecule and predicted lower dissociation temperatures, aligning with experimental data.

Conclusions:

• The novel finite-temperature non-adiabatic molecular dynamics method offers a more robust approach for simulating molecular systems.
• The method accurately captures finite-temperature effects and electron entropy for correlated electronic wave functions.
• Future extensions include incorporating quantum ionic effects and calculating critical temperatures for phase transitions.