A way to optimize atomic position in WIEN2k
The surface structure optimization problem in WIEN2k is an exercise in computational physics in which the surface features of a material are calculated and fine-tuned. WIEN2k is a computer program that is used to estimate the electronic composition of crystals. Adjustments to the WIEN2k calculation may include the lattice parameters (size and design of the unit cell), the alignment of the surface, accuracy of atomic positions, and further parameters in order to determine the most optimal surface structure for a given material. After optimization, WIEN2k is used to assess the characteristics of the surface, allowing scientists to engineer materials suitable for specialized use such as in electronics or energy storage .
By carrying out a structural optimization, we can determine the best position for the anion (x, 0, 0) based on the ionic radii of the cations occupying the A site for various systems (double perovskites, etc.). We can then begin the process by assigning an initial value for x less than 1. Then, we can view the structure and make sure there are no overlapping elements before continuing with the optimization in order to refine the position to the optimal value.
Furthermore, structure files from Crystallographic Open Database (COD) can be employed to generate a structure file through Wien2K, where pre-prepared files are available. Other databases can be referenced, but most require a fee.
The Port method is another method used to optimize the surface structure. It is an iterative method which involves two steps i.e. determination of the energy surface and then optimization of the same energy surface. It can get more accurate structures compared to the MSR1a strategy and hence is preferred over the latter one in many cases.
In order to run a magnetic supercell (MSR1a) calculation with WIEN2k, it is necessary to first create a magnetic supercell in the STRUC and ATOM files. This involves making certain that the original structure and atom positions in the STRUC and ATOM files align with the magnetic supercell. After that, the SPINOR file must be altered to indicate the spins at each site. Next, the EDIFFG must be chosen in the case.in file, as this sets the convergence criteria for the pressure on each atom. Finally, a spin-polarized calculation should be executed via setting the value of ISPIN in the case.in file to 2.
To minimize an energy surface using MSR1a, we first run a crude SCF cycle to bring the structure close to the Born-Oppenheimer surface. With the cross-section parameters, we reach the initialization stage, where we can check for deviation using various levels of RMT reduction. Since too much RMT reduction can result in a framework error, we can use a 2-5% RMT decrement to see if it is beneficial. After initialization, we go to the minimization point interface and run the run_lapw -min command from a terminal. We can then select case.inm and enter the SCF-execution, which optimizes positions and charge density together in a single SCF-run. For this procedure, the typical convergence criteria would be -cc 0.001, -fc 1, and -ec 0.0001. To start the SCF cycle, we click on the Optimize Positions (MSR1a) button. After the cycle has been completed, we can use the utility to display the initial energies and the details of the final relaxation.
Once the MSR1a method is selected, the system is initialized and the energy is calculated. This is then followed by the minimization of the energy of the system using a series of Kohn-Sham calculations. The minimization steps, like the step size and damping factor, are all set accordingly. The process of optimization is continued until all the Kohn-Sham energies converge, which indicates that the optimized energy value of the system has been reached.
Once this is done, the optimized structure is saved and the next step of the optimization process is complete.
The Step by step calculation should be like this:
1. Make structure initially
2.Run crude scf with favourable Rmt, k-points,RKMAX etc
3.Go to the minimize position interface
4.Click on change case.inm and go to the scf-execution.
5.Click on optimize position (MSR1a) with suitable energy and charge convergence criteria like -cc 0.001, -fc 1, -ec 0.0001
6.Run scf
This optimization process can be repeated until the desired optimized energy value is achieved.
Why MSR1a method is really necessary?
The MSR1a method is essential for WIEN2k since it enables users to precisely determine the magnetic properties of materials, including spin polarization, magnetic moments, and exchange constants. Furthermore, the MSR1a method can be used to predict the exchange coupling parameters in ferromagnetic compounds and help users simulate their magnetism accurately. This is especially beneficial when it comes to selecting and designing materials. Additionally, the MSR1a method significantly reduces the amount of computational time given its efficiency compared to other methods usually used in density functional theory deductions.
Sources :
1. https://youtu.be/MyPik2lib5U
3. http://wien2k-algerien1970.blogspot.com/2017/12/how-to-do-geometrical-optimization-of.html?m=1
4. http://wien2k-algerien1970.blogspot.com/2016/09/?m=1
5. https://www.slideshare.net/algerien1970/wien2k-getting-started
Elastic Property Calculation using IRelast
WIEN2k is a highly efficient, full-potential, linearized augmented plane wave (FP-LAPW) program for predicting the electronic and magnetic structure of solids through density functional theory (DFT) calculations. It is capable of calculating a variety of properties of solids, including the cohesive energy, band gaps, phonon frequencies, and lattice constants, but cannot calculate the elastic properties of a material directly. To calculate the elastic properties, methods such as ab initio calculations, model potentials, or empirical force fields can be employed. The Regularly Muffin Tin (RMT) technique also allows for the calculation of the elastic constants of a material, yet does not fully capture important interactions in real materials, therefore leading to errors in the predicted elastic properties.
RMT (Regularly Muffin Tin) is based on the notion that the electrons in a solid move in a periodic, organised manner through a lattice. This enables the calculation of the material's elastic constants, which are integral for its characterisation. Nonetheless, this theory disregards some of the intricate interactions found in real materials, such as spin–orbit coupling or lattice relaxation, which could lead to errors in the elastic property computations.
so, to do elastic property calculation using IRelast-
First, we have to read the part of elast in userguid, and after that we can start your calculation.
To calculate the elastic constants of cubic crystal, you have to put .struct and .outputous files in the calculation directory, then follow the userguid procedures.
If we want to calculate elastic constant of other symmetry unless cubic, we must to use another code such as "Elastic constant" of Morteza Jamel or "Elastic 1.0" and are free in this link: http://www.wien2k.at/reg_user/unsupported/
IRelast: (Version 2022) is a package for finding elastic constants of cubic, hexagonal, orthorhombic, tetragonal, monoclinic and rhombohedral materials . It was developed by Morteza Jamal (m_jamal57@yahoo.com)i and is included in the WIEN2k_22 package. He also provides a couple of videos which can be downloaded here:
example-MgO-under-pressure-5GPa-2022.mkv
This program is a component of the WIEN2k package and is used to calculate the elastic constants of an approximation to a crystal.
To begin the calculation, one should first follow the normal procedure of setting up the WIEN2k calculation -- generating the input file and running the SCF calculation. After the SCF calculation has converged, go to the main directory of WIEN2k and run the IRelast program. The IRelast program will first ask for the lattice constants and other parameters of the crystal. Next, the program will construct the elastic constants of the system based on a linear response function, and the results will be written in a separate file called IRelast.sum.
Finally, the elastic properties are computed from the IRelast.sum file, and the results can be read from the screen or saved in a separate file.
1. Download and install the WIEN2k software package on your system.
2. Set up a computational parameter file using the ‘struct’ program to describe the structure of the material you are studying.
3. Create a new ‘startup’ file for the elastic constant calculation.
4. Compute the elastic constants of the material using the ‘Elast’ program (set_elast_lapw)
5. Use necessary convergence criteria like -ec 0.0001 etc.
6. Use CUBIC.job or Call.job in each folder of elastic constant
7. If no error occurred, then input cal_elast_lapw in the outer directory of elastic constant calculation.
◈ The CUBIC.job command in WIEN2k is used for elastic property calculation. It is an in-built script which performs a lattice constant optimization and a relaxation of internal degrees of freedom for a given crystal structure. The output of the command is a set of elastic constants calculated for the given structure.
◈ The command "Call.job" in the WIEN2k program is used to run an elastic property calculation on a set of crystal structures. This command allows you to calculate elastic constants, strain energies, and other elastic parameters. It can also be used to calculate general strain optimization, as well as static and dynamic saddle points.
◈ The set_elast_lapw command is an abinit command that is used to specify the parameter used to identify the elastic, linear-elastic, and plastic material constants. The command is typically used with Abinit to perform structural, thermodynamic, and linear-response calculations of materials. The parameter includes two parts: the material class, which represents the physical properties of the material, and the material parameters that describe the material's response to external forces. The parameters are specified in terms of stress, strain, and plastic strain. The command allows for the specification of the materials' yield stress, bulk modulus, and shear modulus, as well as the elastic and plastic strains required to cause a given level of deformation.
The most common errors that can occur during elastic property calculations in WIEN2k are numerical errors due to finite basis sets, insufficient Monte Carlo integration, and insufficient number of k-points. These errors can lead to inaccurate results, such as a wrong lattice constant or incorrect phonon frequencies, which can affect the results of the calculation. Other errors that can occur include improper choice of functional (e.g DFT) or pseudopotential, or errors in the input parameters for the calculation. The WIEN2k code is an all-electron full-potential linearized augmented plane wave (LAPW) code for computing ground-state properties of atoms, molecules, and solids. It calculates various elastic properties, such as the elastic constant and stiffness constants.
☒ Styp -2.00 error in one of the elastic properties calculated by WIEN2k suggests that a calculation error occurred. This could be due to incorrect input parameters for the calculation, such as the lattice parameters or temperature, or the input of the wrong functional for the calculation. It could also be due to a bug in the code itself. In either case, the user should double-check the input parameters and parameters used in the calculation and make sure that everything is correct. If the problem persists, the user should contact the developers of WIEN2k to ask for help in troubleshooting the issue.
☒ Styp errors in elastic property calculation in WIEN2k are also caused by elastic property calculation in wrong directoy and numerical approximations used in the calculation of phonon frequencies. The most common approximation used in WIEN2k is inharmonic force constants, which are calculated using the second order perturbation method. This method requires a pre-defined cutoff radius to filter out interactions beyond the cutoff radius, which can lead to errors if the cutoff is too large or too small. Additionally, the use of a k-point mesh for the Brillouin zone integration used to calculate phonon frequencies can also lead to errors if the mesh is too coarse.
More information can be found here:
http://www.wien2k.at/reg_user/unsupported/guide-IRgrace-1.pdf
https://doi.org/10.1016/j.jallcom.2017.10.139
Sources:
http://wien2k-algerien1970.blogspot.com/2016/09/calculation-of-elastic-properties-using_6.html
http://zeus.theochem.tuwien.ac.at/pipermail/wien/2021-September/031947.html
Effective mass calculation using E-K diagram
WIEN2k analyzes the behavior of electrical charges within a crystal using effective mass calculation. The effective mass, which describes how charged carriers behave in a patterned potential, is used in this method. It describes how easily a carrier can move within the crystal structure when an electric field is applied. The technique entails solving the Schrödinger equation for electrons using the crystal potential, then determining the second derivative of energy, which corresponds to the wave vector. The effective mass is then calculated by inverting the second derivative.
The computation of effective mass is crucial in developing and refining electronic gadgets such as solar panels and transistors, as it enhances the comprehension of the electrical attributes and mobility of materials. Generally, effective mass is evaluated by referring to the E-K chart. The solid-state physics employs an E-K diagram or energy-momentum diagram or dispersion diagram to demonstrate the energy and momentum of electrons in a crystal composition.
The diagram depicts the correlation between energy (E) and momentum (k) of allowed electronic states in a crystal. The vertically arranged y-axis represents energy levels of electrons that are generally measured in electron volts, while the horizontally arranged x-axis represents the magnitude of momentum, which can be either negative or positive (eV).
The E-K diagram illustrates the band structure of a crystal which is defined by the allowed electronic states within its lattice. At absolute zero, all electrons occupy the valence band, the lowest energy state. However, as temperature rises, some electrons acquire enough energy to move into the conduction band, which is a higher energy level where they can move freely through the crystal and conduct electricity.
The difference between the valence and conduction bands is represented as the bandgap, which determines the electrical behavior of the material. A conductor has overlapping valence and conduction bands that allow easy electron flow, while a semiconductor has a small bandgap that can be overcome by temperature or doping. In contrast, an insulator has a large bandgap that prevents electron movement and electrical conduction.
Moreover, the E-K diagram also displays how materials interact with photons of diverse wavelengths. This feature aids in understanding the properties of optoelectronic devices like solar cells and LEDs.
To explain the electronic properties of solids, professionals use the E-K diagram that has two axes, where the x-axis displays the momentum while the y-axis represents the energy level of electrons or particles. The E-K diagram can become multidimensional depending on the energy levels and momentum directions. The diagram indicates various bands of energy levels that correspond to electronic states in the crystal lattice, with solid lines indicating allowed energy levels and dotted lines indicating forbidden levels that electrons can't occupy. The highest occupied energy level where electrons reside at zero temperature, known as the Fermi level, is shown as a horizontal line. The momentum axis displays allowed momentum values in crystalline solids that are quantized, resulting in discrete bands on the E-K diagram, with each band representing different momentum values. Consequently, the E-K diagram is a useful tool helpful for experts in comprehending electronic qualities of solids that facilitate the designing and creation of new electronic devices and materials.
Here are the steps to calculate the effective mass using E-K diagram:
1. Plot the energy-momentum (E-K) diagram of the material you are interested in. This diagram shows the relationship between energy and momentum of the electrons in the material.
2. Locate the band structure in the E-K diagram that you want to calculate the effective mass for. This band structure usually refers to the valence band or the conduction band.
3. Find the curvature of the band structure. The curvature is the rate of change of the energy with respect to the momentum, which can be calculated by taking the second derivative of the energy with respect to the momentum. The curvature is usually denoted as d^2E/dK^2.
4. Calculate the effective mass using the curvature formula. The effective mass can be calculated using the following formula:
m = ℏ^2/ (d^2E/dK^2)
where m is the effective mass, ℏ is the reduced Planck constant, and d^2E/dK^2 is the curvature.
5. Use the appropriate units. The effective mass is usually expressed in terms of the electron rest mass, me, which is 9.11 × 10^-31 kg. Therefore, the effective mass should be divided by the electron rest mass to get the effective mass in terms of the number of electron masses.
Note: When analyzing the E-K diagram, the effective mass is determined as an average mass considering the complicated electron wave function in the material. This average value can vary in different directions due to anisotropy but is generally taken as an overall average. However, this approach is based on the assumption of a quadratic dispersion relation. In the case of non-quadratic dispersion relations, the effective mass may need to be calculated through numerical methods.
Sources:
4..Effective mass in semiconductors (miun.se)