Mengqi Cheng 
(ICTP)

Abstract:
In this talk, I present our work on atomistic simulations of materials under extreme temperature, pressure, and anisotropic stress. I first introduce a constrained ab initio molecular dynamics (CAIMD) framework that enables simulations under non-uniform stress conditions, including uniaxial compression and shear. Building on this, I incorporate an on-the-fly machine learning scheme into CAIMD, which significantly improves computational efficiency while maintaining first-principles accuracy and enables continuous generation of high-quality training data. I also outline ongoing efforts toward a DeepMD-based ML-CAIMD framework for larger-scale simulations.
Using this approach, I investigate the high-temperature stability and melting behavior of cubic boron arsenide, revealing a reentrant melting behavior linked to structural changes in the liquid phase. I further extend the method to disordered systems and perform large-scale simulations of glassy carbon under anisotropic stress, identifying new transformation pathways toward quenchable amorphous diamond and highlighting the key role of shear in stabilizing sp\textsuperscript{3} bonding.
Finally, I discuss its application to superionic water. Recent experiments have reported a mixed close-packed structure in the superionic regime, but it remains unclear whether the observed stacking disorder originates from strong shock-driven uniaxial, high-strain-rate compression or represents an intrinsic feature of superionic ice. This open question motivates simulations under combined shear and compression, which are directly accessible within the present framework and relevant to realistic planetary conditions.

Ref.
1. Mengqi Cheng and Hong Sun, Physical Review Materials 8, 113604 (2024), “Stability of c-BAs under extreme conditions: An ab initio molecular dynamics study.”
2. Mengqi Cheng, Weidong Luo, and Hong Sun, Physical Review B 112, 094207 (2025), “On-the-fly machine learning augmented constrained ab initio molecular dynamics to design routes from glassy carbon to quenchable amorphous diamond.”
3. L. Andriambariarijaona et al., Nature Communications 17, 374 (2026) “Observation of a mixed close-packed structure in superionic water.”

Zoom registration link:
https://zoom.us/meeting/register/SHbwboMjRqOBISPZF0dcEA