Tips for this lab and PWSCF in general
Problem 1 will look at calculations involving cobalt. We will use the LDA exchange-correlation functional. Since Co is a ferromagnetic material, we will do spin-polarized calculations. To set up these calculations, you should have a basic knowledge of crystal structures. Chapter 1 of Introduction to Solid State Physics by Kittel is a good introduction. Additional points on the input file that were not discussed for Lab 2:
- ibrav=4 refers to the hexagonal lattice
- There are two independent lattice parameters in a hexagonal lattice (as you probably know). celldm(1) is a in bohr and celldm(3) is c/a (not the absolute value of c!). Find an appropriate value of celldm(3) from your knowledge of the ideal HCP structure. Two atoms comprise one unit cell.
- We are going to use ultrasoft pseudopotentials here. In the case of norm-conserving pseudopotentials from Lab 2, ecutrho (the charge density cutoff) is automatically determined by 4*ecutwfc. However, in the case of ultrasoft pseudopotentials, we need an augmented charge around the ion core, so ecutrho should be higher than 4*ecutwfc. The usual value is 25-35 Ry for ecutwfc and 200-300 Ry for ecutrho. You might want to do a convergence check. Keep in mind that the value you should look at is the energy difference or force, not the absolute value of the energy (the energy will not converge unless you use very, very high ecutwfc and ecutrho).
- Because Co is a magnetic element, we have to run spin polarized calculations. starting_magnetization is the starting magnetization for the atom. Set this to a value between -1 and +1.
- occupations, degauss, smearing: These keywords are particular details for the Brillioun zone integration for metals. Since there is a discontinuity of the occupation number for the bands around the Fermi energy, total energy with respect to the number of k-points converges very slowly. Adding electronic temperature (degauss) smooth out the abrupt change of the occupation number and as a result total energy converges with fewer number of k-points.
- nspin=2 turns ON spin polarization while nspin=1 turns it OFF. We will use nspin=2 throughout Problem 1.
Discussion of some parts of the Problem 2 input file:
- When you are asked to relax the atoms in BaTiO3, instead of a single self-consistent field calculation ('scf'), we will be doing a 'relax' calculation which includes a series of SCF calculations. Here, the ions are allowed to move in order to reduce the total system energy. This setting is very much like setting the 'opti' flag in GULP.
- Since we will be using ion “dynamics”, we now need the new IONS section. We are not, however, using real dynamics (i.e. there is no time coordinate used in the relaxations), but just searching for the minimum energy relaxations. This section should be omitted for the scf calculations of part (a).