Structural and electronic properties of semiconductors and metals

Prev: LabQSM#Lecture 1: Basic DFT calculations and Convergences

Structural and electronic properties of Diamond

In this tutorial we will see how to setup a calculation and to get total energies using the PW code from the Quantum ESPRESSO distribution.

Some helpful conversions:

1 bohr = 1 a.u. (atomic unit) = 0.529177249 angstroms.

1 Rydberg = 13.6056981 eV

1 eV =1.60217733 x 10-19 Joules

For all first-principles calculations, you must pay attention to two convergence parameters. The first one is the energy cutoff, which is the max kinetic energy used in wave-function expansion. The second is the number of k-points, which measures how well the continuous integral over the BZ is discretized.

Diamond is a face-centered cubic structure with two C atoms at 0 0 0 and 0.25 0.25 0.25 a is the lattice parameter

Now let's see in detail how a QE input is structured to make a total energy calculation for this system. An example can be found in ~/LabQSM/LAB_1/test_diamond/scf.diamond.in that you can read e.g. using the editor vi:

Input file description

&CONTROL
   prefix='diamond',
   calculation = 'scf'
   restart_mode='from_scratch',
   pseudo_dir = './pseudo/'
   outdir = './SCRATCH'
/
&SYSTEM
   ibrav = 2,
   celldm(1) = 6.7402778
   nat = 2,
   ntyp = 1,
   ecutwfc =  40.0
/
&ELECTRONS
   mixing_mode = 'plain'
   mixing_beta = 0.7
   conv_thr =  1.0d-8
/
ATOMIC_SPECIES
 C   12.011   C.pz-vbc.UPF
ATOMIC_POSITIONS (alat)
 C 0.00 0.00 0.00
 C 0.25 0.25 0.25 
K_POINTS {automatic}
 4 4 4   0 0 0

The input file for PWscf is structured in a number of NAMELISTS and INPUT CARDS:

&NAMELIST1 ... /
&NAMELIST2 ... /
&NAMELIST3 ... /
INPUT_CARD1
 ....
 ....
INPUT_CARD2
.... 
....

The use of NAMELISTS allows to specify the value of an input variable, not all the variables need to be specified, for most variable, a default value is assigned. Variable can be inserted in any order. NAMELISTS are read in a specific order. INPUT CARDS are specific of QuantumESPRESSO codes and are used to provide input data that are always needed.

There are three mandatory NAMELISTS:

• &CONTROL: variables that control the kind of calculation (here scf), where to get the pseudopopotentials, and the verbosity needed in the output.
• &SYSTEM: variables specifying the system as the crystal structure, number of atoms, dimension of the basis set.
• &ELECTRONS variables controlling the algorithm to solve the Kohn-Sham equations.

Next, we have three mandatory INPUT CARDS:

• ATOMIC_SPECIES : name, mass and pseudopotential file
• ATOMIC_POSITIONS: atom type and coordinate in the unit cell
• K_POINTS: definition of the k point grid for the BZ integration

The complete list of QE variables can be found in the documentation of the code at: | http://www.quantum-espresso.org/Doc/INPUT_PW.html