# [Pw_forum] Exceptionally large charge density cut-off

Sat Mar 29 18:16:37 CET 2008

```I am trying to calculate phonons for a L10 structure with c/a=0.96. The
system is magnetic FePd. While trying to having an estimate of the
relevent parameters of the calculation(kinetic energy cut-off, charge
density cut-off, k-mesh etc) I find that phonon frequencies at Gamma and
[1/2 1/2 1/2] points do not converge even after a large charge density
cut-off although with respect to k-mesh size and kinetic energy cut-off,
the convergence was achieved with reaonable values of these parameters.

I try to converge the phonon frequencies to ~1% for each of these
parameters. The non-converegence to within 1% with respect to charge
density cutoff surprises me. Can somebody help?

Following is the frequencies I obtain:

1) Gamma point:

I got convergence ~1% for k-mesh and kinetic energy cutoff with smearing
parameter=0.04 Ry. For these two parts charge density cutoff was kept
fixed at 900 Ryd. I got convergence with a k-mesh=24x24x17 and kinetic
energy cutoff=55 Ry.

When I vary charge density cutoff this is what I get:

rhocutoff              Frequencies:
900                   4.01,7.97,162.42, 229.66
1000                   7.35, 11.21,162.62,229.61
1100                    5.31, 6.72, 162.14,229.5
1200                    11.53 12.48 162.68 230.09
1300                      5.30 6.55 162.04 229.67
1400                     7.79 10.53 162.52 229.72
1500                      6.68 7.12 162.12 229.64
1600                      6.87 7.39 162.21 229.70
1700                      7.89 9.56 162.38 229.73
2000                      8.99 10.59 162.52 229.79

The optical modes are well converged but the acoustic modes are not
converged to within 1%, rather are oscillating.

2) [1/2 1/2 1/2] point

Here, kinetic energy cut-off is 45 Ry. The results for various rhocutoff
values are as follows:

rhocut                     frequencies
900            65.30 90.99 130.26 139.80 207.39 247.39
1000           66.71 90.22 131.71 140.84 208.36 247.56
1100           63.17 89.93 129.08 138.94 206.49 246.89
1200           68.42 91.14 132.47 142.14 209.08 248.19
1300           64.18 90.76 129.24 139.39 206.73 247.17
1400           66.99 91.02 131.54 140.94 208.34 247.76
1500           66.33 90.92 130.56 140.64 207.88 247.62
1600           64.80 91.13 129.46 139.56 206.99 247.31
1700           67.42 91.04 131.55 141.22 208.53 247.86
1800           67.30 90.95 131.58 141.16 208.82 247.82
2000           65.48 90.93 130.52 140.18 207.45 247.41

The lowest lying acoustic mode is not yet converged to within 1% while the
other modes have converged.

This is very surprising because when I looked into the archive and the
relevent literature I didn't find any reference to such a behavior.

What could be the possible reason?

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Department of Physics
Indian Institute of Technology
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