Extraordinary negative thermal expansion of two-dimensional nitrides: A comparative <i>ab initio</i> study of quasiharmonic approximation and molecular dynamics simulations

Thermal expansion behavior of two-dimensional (2D) nitrides and graphene were studied by ab initio molecular dynamics (MD) simulations as well quasiharmonic approximation (QHA). Anharmonicity the acoustic phonon modes are related to unusual negative thermal (NTE) nitrides. Our results also hint that direct MD a more elaborate method investigate 2D materials than QHA. Nevertheless, giant NTE coefficients found for $h$-GaN $h$-AlN within covered temperature range 100--600 K regardless chosen computational method. This is reasoned with out-of-plane oscillations rippling monolayers.