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  • Finite temperature calculations

Finite temperature calculations · Changes

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Update Finite temperature calculations authored Feb 19, 2026 by Gabriel Wlazłowski's avatar Gabriel Wlazłowski
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Finite-temperature-calculations.md
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W-SLDA Toolkit allows for calculations at finite temperature. Here we describe how the temperature effects are implemented in the toolkit. We emphasize that at conceptual level **some of assumptions may be incorrect**.
The W-SLDA Toolkit allows calculations at finite temperature. Here we describe how the temperature effects are implemented in the toolkit. We emphasize that at the conceptual level **some of the assumptions may be incorrect**.
# Static calculations
Finite temperature effects are introduced by adding quasiparticle occupation probabilities in form of Fermi-Dirac function:
Finite temperature effects are introduced by adding quasiparticle occupation probabilities in the form of the Fermi-Dirac function:
```math
f_{\beta}(E_n)=\dfrac{1}{\exp(\beta E_n)+1}
```
to definition of densities (see [here](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/wikis/Physical%20quantities#densities) from explicit formulas). Here $`\beta=1/T`$ is inverse of temperature. In zero temperature limit this function reduced to step function. Quasiparticle energies $`E_n`$ are taken from solution of Bogoliubov-de Gennes type equations
to definition of densities (see [here](Physical-quantities#densities) from explicit formulas). Here $`\beta=1/T`$ is the inverse of temperature. In the zero temperature limit, this function reduces to a step function. Quasiparticle energies $`E_n`$ are taken from the solution of Bogoliubov-de Gennes type equations
```math
H_{\textrm{BdG}} \begin{pmatrix}u_{n\uparrow}(r) \\ v_{n\downarrow}(r)\end{pmatrix}= E_n\begin{pmatrix}u_{n\uparrow}(r) \\ v_{n\downarrow}(r)\end{pmatrix}
H_{\textrm{BdG}} \begin{pmatrix}u_{n,a}(r) \\ v_{n,b}(r)\end{pmatrix}= E_n\begin{pmatrix}u_{n,a}(r) \\ v_{n,b}(r)\end{pmatrix}
```
See [here](https://gitlab.fizyka.pw.edu.pl/wtools/wslda/-/wikis/Physical%20quantities#potentials) from explicit form of $`H_{\textrm{BdG}}`$.
See [here](Physical-quantities#potentials) from explicit form of $`H_{\textrm{BdG}}`$.
Presently, **definition of densities is the only place where temperature enters into calculation process**. In general finite temperature DFT allows energy density functional $`\mathcal{E}_{\textrm{edf}}`$ to be temperature dependent, which is not the case neither (A)SLDA or BdG functionals.
Presently, the **definition of densities is the only place where temperature enters into the calculation process**. In general, finite temperature DFT allows energy density functional $`\mathcal{E}_{\textrm{edf}}`$ to be temperature dependent, which is not the case for either (A)SLDA or SLDAE functionals.
Below we provide collection of predictions of ASLDA functional when applied to 3D uniform and spin-symmetric system. Figures are taken from diploma thesis of [Aleksandra Olejak, WUT, 2017
Below, we present a collection of predictions from ASLDA for functional when applied to 3D uniform and spin-symmetric systems. Figures are taken from the diploma thesis of [Aleksandra Olejak, WUT, 2017
](https://apd.usos.pw.edu.pl/diplomas/18015/).
![ASLD-finiteT](uploads/4b58c5cd64ad2a853ef30304c861a508/ASLD-finiteT.png)
From these plots it is clearly seen that critical temperature is located around $`T_c\approx0.3\varepsilon_F`$ whereas experimentally measured critical temperature is $`T_c^{(\textrm{exp})}\approx0.15\varepsilon_F`$. It clearly demonstrates that present formulation of ASLDA can only at most simulate finite temperature effects at qualitative level.
From these plots, it is clearly seen that the critical temperature is located around $`T_c\approx0.3\varepsilon_F`$, whereas the experimentally measured critical temperature is $`T_c^{(\textrm{exp})}\approx0.15\varepsilon_F`$. It clearly demonstrates that the current formulation of ASLDA can at best simulate finite-temperature effects at a qualitative level.
In order to activate finite temperature calculations you need to uncomment in input *file*:
In order to activate finite temperature calculations, you need to uncomment in the input *file*:
```bash
temperature 0.1 # requested temperature in units of eF, default T=0
```
*Recommendation*: For numerical purposes it is convenient to introduce a very small temperature (much smaller than any other energy scale in the system). It greatly improves convergence properties of algorithm, especially in context of spin-imbalanced systems.
*Recommendation*: For numerical purposes, it is convenient to introduce a very small temperature (much smaller than any other energy scale in the system). It greatly improves the convergence properties of the algorithm, especially in spin-imbalanced systems.
# Time dependent calculations
**Conceptually correct introduction of temperature effects into time-dependent calculations is still an open question**. For integrity purposes of W-SLDA Toolkit the *temperature effects* are introduced to `td-wslda` codes as follow:
1. all densities are calculated with included Fermi-Dirac function, for example:
# Time-dependent calculations
**Conceptually correct introduction of temperature effects into time-dependent calculations is still an open question**. For integrity purposes of the W-SLDA Toolkit, the *temperature effects* are introduced to `td-wslda` codes as follows:
1. All densities are calculated with the included Fermi-Dirac function, for example:
```math
n_{\uparrow}(\bm{r},t)= \sum_{|E_n|<E_c}|u_{n,\uparrow}(\bm{r},t)|^2 f_{\beta}(E_n)
n_{a}(\bm{r},t)= \sum_{|E_n|<E_c}|u_{n,a}(\bm{r},t)|^2 f_{\beta}(E_n)
```
2. values of $`E_n`$ are taken from initial (static) solution and keep frozen over entire time evolution.
2. values of $`E_n`$ are taken from the initial (static) solution and kept frozen over the entire time evolution.
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