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Computational Method
This protocol is extracted from research article:
Manipulation of Stacking Order in Td-WTe2 by Ultrafast Optical Excitation
ACS Nano, Apr 29, 2021; DOI: 10.1021/acsnano.1c01301

The band-structure and density of states presented in Figure Figure66 and the structural dependence on hole-doping presented in Figure Figure77 were calculated using the VASP implementation of DFT.41,42 Exchange and correlation was treated with the GGA functional by Perdew, Burke, and Ernzerhof43 and with Grimme’s D3 treatment of dispersion interaction,44 including Becke-Johnson damping.45 We used an energy cutoff for the plane-wave expansion of 446 eV. The Brillouin-zone was sampled with 12 × 6 × 3 k-points in a Γ-centered mesh. Spin–orbit coupling is included. Structural relaxations were stopped when the difference in total free energy between two relaxation steps was less than 1e–6 eV.

We performed nonadiabatic molecular dynamics using time-dependent density-functional theory in conjunction with Ehrenfest dynamics as implemented in the TDAP package,46 built on top of the Siesta electronic structure code.47 The simulations were performed using a supercell of 4 × 2 × 1 unit-cells of Td-WTe2, comprising 96 atoms, sampling only the Γ-point of the supercell. The exchange-correlation energy was described within the adiabatic GGA approximation, according to Perdew, Burke, and Ernzerhof.43 As basis set a double-ζ + polarization orbital representation was used. The mesh cutoff for grid-integration was set to 129 Ry. Time-propagation was performed with a 6.045 atto-second time-step, the ionic motion is propagated within the Ehrenfest molecular dynamics scheme using the Verlet algorithm.48 The electromagnetic field of the pump-pulse is represented as a vector potential in the velocity gauge. The external field, represented in the velocity gauge, enters the calculation as a vector potential A(t) = −An external file that holds a picture, illustration, etc.
Object name is nn1c01301_m001.gift0tsin(ωt′) dt′, where An external file that holds a picture, illustration, etc.
Object name is nn1c01301_m002.gif = 0.015 V/Å, and the angular frequency corresponds to ℏω = 2.4 eV. Note that this represents a rectangular pulse shape, in our simulations represented as a continuous pulse starting from t = t0 and ending at t = tend; that is, no Gaussian envelope is applied. See the Supporting Information for details on power density and intensity in relation to a Gaussian envelope. We calculate the time-evolution of the 16 individual Δ1 and Δ2, as defined by the b axis displacement of Te atoms in adjacent layers. Average quantities are calculated according to ⟨Δ1(t)⟩ = 1/N∑Δ1i(t) and ⟨Δ2(t)⟩ = 1/N∑Δ2i(t) (see Figure Figure11 for definition) represented in the supercell.

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