Structural and electronic properties of semiconductors and metals

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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:

&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 only when it is needed and to define default values for most variables that then need not be specified. Variable can be inserted in any order. NAMELISTS are read in a specific order.