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  • Starting ABM algorithm

Starting ABM algorithm · Changes

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Update Starting ABM algorithm authored Mar 19, 2024 by Gabriel Wlazłowski's avatar Gabriel Wlazłowski
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Starting-ABM-algorithm.md
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To start the [ABM algorithm](Integration scheme) one needs to initialize
To start the [ABM algorithm](Integration scheme), one needs to initialize
```math
f(y,t) \equiv \dfrac{1}{i}(\hat{H}(\{\varphi_n\},t)-\langle H\rangle_n)\varphi_n(\bm{r},t)
```
for four time steps back: $`f_{k-1}`$, $`f_{k-2}`$, $`f_{k-3}`$, $`f_{k-4}`$.
# Starting from a stationary state
In the case of a stationary state, $`\varphi_n`$ are eigenstates of the hamiltonian, and the instantaneous quasi-particle energies are equal to quasiparticle energies $`E_n=\textrm{const}`$. Then, as long as the hamiltonian does not depend on the time we obtain:
In the case of a stationary state, $`\varphi_n`$ are eigenstates of the Hamiltonian, and the instantaneous quasi-particle energies are equal to quasiparticle energies $`E_n=\textrm{const}`$. Then, as long as the Hamiltonian does not depend on the time we obtain:
```math
f_{k-1}=f_{k-2}=f_{k-3}=f_{k-4}=0
```
To use this prescription for initialization of ABM algorithm, you need to select in the input file:
To use this prescription for the initialization of the ABM algorithm, you need to select in the input file:
```bash
selfstart 0 # use Taylor expansion for the first 5 steps?
```
# Starting from a non-stationary state
In this case, we need to generate the first five steps using single step method. In case of `td-wslda` we use a method based Taylor expansion of the evolution:
In this case, we need to generate the first five steps using single-step method. In the case of `td-wslda`, we use a method based on Taylor expansion of the evolution operator:
```math
\varphi_n(\bm{r},t+\Delta t) = e^{-i(\hat{H}(\{\varphi_n\},t)-\langle H\rangle_n)\Delta t}\varphi_n(\bm{r},t)\approx \sum_{k=0}^{4}\frac{(-i\Delta t)^k}{k!}(\hat{H}(\{\varphi_n\},t)-\langle H\rangle_n)^k\varphi_n(\bm{r},t)
```
......
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