SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms), a widely used Density Functional Theory (DFT) code for simulating and analyzing materials at the atomic scale. It is used for studying systems with a large number of atoms and helpful in doing simulations in various fields viz. condensed matter physics, materials science, nanoscience, biomolecules, molecular electronics.
Here are some key features and characteristics of Siesta:
Basis Set: SIESTA employs a numerical atomic orbital basis set. Due to which it is efficient for doing calculations on large systems. It uses a localized basis set, such as atomic orbitals, to describe the electronic wave functions.
Pseudopotentials: It employs norm-conserving pseudopotentials to describe the electron-ion interaction. The use of pseudopotential reduces the computational cost because core electrons are effectively replaced with simpler and softer potential and hence in place of actual wave function we will pseudo wave function and smaller basis set in needed to describe such pseudo wave function. Using ATOM code we can generate own pseudopotentials of various atomic species.
Exchange-Correlation Functionals: This code supports a wide range of exchange-correlation functionals, including local density approximation (LDA), generalized gradient approximation (GGA), and hybrid functionals. These functionals account for the exchange and correlation effects of the electron-electron interaction.
Geometry Optimization: Siesta allows for the optimization of atomic positions to find the most stable configuration of a system. It can perform structural relaxation and molecular dynamics simulations to study the dynamics and stability of materials.
Structural, electronic, magnetic and dielectric Properties: Various tools are available in SIESTA. These tools are used to determine structural properties (viz. bond length, bond angle, binding energy, thermal stability (phonon calculations)), density of states (DOS (total DOS and Projected DOS)), spin resolved electronic band structure, magnetic moment, charge transfer (Mulliken charge population and charge density plots), dielectric properties (real and imaginary part of dielectric function, reflectance, absorption spectra, loss function, conductance, refractive index etc.) These properties help us to understand the behavior of electrons and their interactions within the material.
Flexibility of SIESTA: The flexibility of SIESTA allows researchers to study a wide range of dimensional materials (0D, 1D, 2D and 3D due the use of numerical atomic orbitals as basis set), providing insights into their electronic, structural, and vibrational properties. On the basis of dimensionality, materials can be classified as:
0D Materials: SIESTA can be used to investigate zero-dimensional materials such as nanoparticles, clusters, and quantum dots. These systems can be modeled by specifying the atomic positions and sizes of the nanoparticles in the simulation cell.
1D Materials: SIESTA can simulate one-dimensional materials like nanowires, nanotubes, and atomic chains. These systems can be modeled by creating a supercell with periodic boundary conditions along one direction, representing an infinitely long structure.
2D Materials: SIESTA is well-suited for studying two-dimensional materials like graphene, transition metal dichalcogenides (TMDs), and other 2D layered structures. These systems can be modeled by using a 2D periodic boundary condition, representing an infinitely extended sheet.
3D Materials: SIESTA can also simulate bulk materials, including crystals and amorphous structures. In these cases, a 3D periodic boundary condition is applied, representing an infinite lattice in all three dimensions.
Parallelization: Parallel installation of SIESTA enables efficient calculations on high-performance computing clusters. This allows faster simulations by distributing the workload among multiple processors or nodes.
Thermal and electronic transport using TranSIESTA: TranSIESTA is an extension of the SIESTA code that allows for the calculation of thermal and electronic transport properties in materials.
Thermal Transport: TranSIESTA enables the calculation of thermal transport properties, such as thermal conductance and Seebeck coefficient, using the Landauer-Büttiker formalism. It calculates the transmission coefficients for phonons and uses them to determine the thermal conductance of the system. By applying a temperature gradient ane can estimate Seebeck coefficient, which characterizes the thermoelectric properties of the material.
Electronic Transport: TranSIESTA allows for the calculation of electronic transport properties, such as conductance, current-voltage characteristics, and density of states (DOS) of nanoscale devices. It uses the NEGF method to calculate the transmission coefficients for electrons and determines the conductance of the system. By applying a bias voltage, current-voltage characteristics can be obtained, providing insights into the device's electrical properties.
Siesta has been widely used by researchers worldwide to investigate a broad range of materials, including surfaces, interfaces, nanoparticles, and biomolecules. It is open-source software, freely available for academic and research purposes.
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