# Atomistic Investigation of Plastic Deformation and Dislocation Motion in Uranium Mononitride
## 2 Computational Methods
### Dislocation Motion
The issue likely stems from repulsion between uranium atoms, as the Kocevski potential cannot stabilize metallic uranium [12]. As a result, only the Tseplyaev potential was used to study edge dislocations. Both the Kocevski and Tseplyaev potentials could stabilize screw dislocations. However, when simulating the motion of the screw dislocation with the Tseplyaev potential, a trail of vacancy and interstitial clusters continuously formed during dislocation movement, even at low stresses. In contrast, the \(\frac{1}{2}[110](\overline{1}10)\) screw dislocation moved smoothly up to about 1125 MPa under the Kocevski potential. Therefore, the screw dislocation was studied exclusively with the Kocevski potential. The Peierls stress is the applied resolved shear stress that moves a dislocation by at least one Burgers vector without thermal assistance, typically near \(T=0\) K [32]. This is usually measured under a condition of constant strain increment [14; 36]. However, defining strain increments at 0 K is somewhat artificial. Our previous study [12] suggests  
Figure 1: **(a)** Schematic of the supercell constructed to study straight edge dislocations. For glide along the \(\{110\}\) planes, \(x=[110]\), \(y=[1\bar{1}0]\), and \(z=[001]\). For glide along the \(\{111\}\) planes, \(x=[\bar{1}10]\), \(y=[111]\), and \(z=[\bar{1}\bar{1}2]\). The top layer on which stress/strain is applied, as well as the fixed bottom layer, is also shown. **(b)** OVITO visualization of the \(\frac{1}{2}[\bar{1}10](111)\) edge dislocation and its Burgers vector after equilibration at 1 K. The free surfaces can be identified from the gray “defect mesh”. In **(a)** the base plane is the \(xz\) plane, whereas in **(b)** the base plane is the \(xy\) plane. that the Tseplyaev potential may predict metastable states for UN supercells strained at 0 K. To avoid these issues, we calculate the Peierls stress at \(T=1\) K using both potentials. After static minimization, the supercell containing a dislocation is equilibrated in the _NVT_ ensemble for 50 ps. To eliminate the impact of thermostat friction, the thermostat function is simulated with the microcanonical (_NVE_) ensemble coupled to velocity scaling. Once equilibration is complete, velocity scaling is turned off. Shear strain rates \(\dot{\epsilon}_{xy}\) and \(\dot{\epsilon}_{yz}\), both set to \(10^{-4}\) s\({}^{-1}\), are then applied to the top layer of the supercell, containing an edge and screw dislocation, respectively. [PERSON] and [PERSON] [14] demonstrated that the Peierls stress is independent of strain rate. The simulation runs for 2 ns, and the stress on the dislocation is measured by averaging the atomic \(\sigma_{xy}\) and \(\sigma_{yz}\) stresses for the edge and screw dislocations, respectively, every 10 ps. The dislocation mobility function is the essential input parameter in any plasticity or dislocation dynamics model [19]. The dislocation mobility of UN at 300 K is computed by first scaling the relaxed supercells by the ratio of the 0 K and 300 K lattice parameters after relaxation to correct for thermal expansion [16]. Then, the system is equilibrated at a temperature of 300 K under the _NVE_ ensemble coupled to velocity scaling for 50 ps. After equilibration, a shear force is applied to the upper layer of the supercell, whose thickness is 0.05 \(l_{y}\). The simulation is run for 1 ns and the position of the moving dislocation is recorded every 10 ps. The calculation is repeated using 5 different initial velocity distributions for both dislocation types. The shear stress for edge dislocations varies between 25 and 2250 MPa, while for screw dislocations, it ranges from 25 to 1200 MPa, with both cases using a stress increment of 25 MPa. For all calculations, dislocation motion is tracked using the following methodology: Snapshots of the atomic trajectories of each calculation are processed using OVITO where, for each snapshot, the location of the dislocation is identified using the CSP algorithm.