Doubt with interatomic force constant analysis

Running Abinit Response calculations
1

Hi everyone,

I was trying to understand the meaning of the anaddb output for the inter atomic force constants values. I was doing the procedures similar to the DFPT2 tutorial to get phonon dispersion of Rutile TiO2 sytem with the following configuration:

acell 8.7240346929E+00 8.7240346929E+00 5.5299933164
rprim 1.000000000000000 0.000000000000000 0.000000000000000
0.000000000000000 1.000000000000000 0.000000000000000
0.000000000000000 0.000000000000000 1.0000000000000

natom 6
ntypat 2
typat 1 1 2 2 2 2
znucl 22 8

xred 0.0000000000E+00 0.0000000000E+00 5.0000000000E-01
5.0000000000E-01 5.0000000000E-01 0.0000000000E+00
1.9521287849E-01 8.0478712151E-01 0.0000000000E+00
3.0478712151E-01 3.0478712151E-01 5.0000000000E-01
8.0478712151E-01 1.9521287849E-01 0.0000000000E+00
6.9521287849E-01 6.9521287849E-01 5.0000000000E-01

For the anaddb part, giving the following settings:

ifcflag 1 ! Interatomic force constant flag

!Wavevector grid number 1 (coarse grid, from DDB)
brav 1 ! Bravais Lattice : 1-S.C., 2-F.C., 3-B.C., 4-Hex.)
ngqpt 4 4 4 ! Monkhorst-Pack indices
nqshft 1 ! number of q-points in repeated basic q-cell
q1shft 3*0.0

!Interatomic force constant info
dipdip 1 ! Dipole-dipole interaction treatment
ifcana 1 ! Analysis of the IFCs
ifcout 20 ! Number of IFC’s written in the output, per atom
natifc 1 ! Number of atoms in the cell for which ifc’s are analysed
atifc 1 ! List of atoms

gave me the following output:

generic atom number 1
with cartesian coordinates 0.00000000E+00 0.00000000E+00 2.76499666E+00

Third atom defining local coordinates :
ib = 5 irpt = 63,

followed by the 20 different interactions for atom 1 as i specified.
Since it takes unit cell 63 as the reference point, I get the point of looking at atoms in cells 63,64,62,etc to capture the nearest neighbors interactions. But I don’t get why the interactions with atoms in cells 38,33,34, etc. Are those cells too far from the anchor point?
Also are these cells under consideration chosen in random?

Can someone help me with getting these concepts clearer?

Thanks
Dominic

2

Hi Dominik,

The canonical coordinates rcan are ordered a bit randomly. They fill up a huge box, are truncated by a Wigner Seitz construction, and further by the radius you choose in the input file, to decide which ifc to print. The third atom is needed to orient the tensor around the line from atom i to j.

You should have the amplitude of the ifc as a fonction of distance, that’s usually the most sensible thing to plot, to show long and short range effects

MV

3

Hi mverstra,

Thanks for your response. I was able to make the plots with abipy.dfpt.ifc. I used the get_ifc_cartesian(atom_element=“Ti”, neighbour_element=“O”) to get the distances and the cartesian IFC matrices.
I used the [0,0] element in the IFC matrix for the longitudinal response and the [1,1] and [2,2] elements for the transverse responses.

Can we manually do something like setting the IFCs beyond say 30 Bohr to zero and plot the corresponding phonon dispersion and see a comparison between this and the original one?

Thanks
Dominic