  Two-dimensional (2D) metal-organic frameworks (MOFs) hold immense potential
for various applications due to their distinctive intrinsic properties compared
to their 3D analogues. Herein, we designed in silico a highly stable
NiF$_2$(pyrazine)$_2$ 2D MOF with a two-periodic wine-rack architecture.
Extensive first-principles calculations and Molecular Dynamics simulations
based on a newly developed machine learning potential (MLP) revealed that this
2D MOF exhibits huge in-plane Poisson's ratio anisotropy. This results into an
anomalous negative in-plane stretchability, as evidenced by an uncommon
decrease of its in-plane area upon the application of uniaxial tensile strain
that makes this 2D MOF particularly attractive for flexible wearable
electronics and ultra-thin sensor applications. We further demonstrated that
the derived MLP offers a unique opportunity to effectively anticipate the
finite temperature mechanical properties of MOFs at large scale. As a
proof-concept, MLP-based Molecular Dynamics simulations were successfully
achieved on 2D NiF$_2$(pyrazine)$_2$ with a dimension of 28.2$\times$28.2
nm$^2$ relevant to the length scale experimentally attainable for the
fabrication of MOF film.
