[Lammps-user-discuss] negative values dynamical matrix

[enrique m. <enr...@gm...>]

Hello,

When following the example for the dynamical_matrix command (here
<https://github.com/lammps/lammps/tree/develop/examples/PACKAGES/phonon/dynamical_matrix_command/Silicon>)
and turning on the minimization flag, there are substantial negative
values. Here are the first six lines of the output file that’s generated
when I run it:

5407.73126648 0.60396688 0.00000175
-1276.30761693 -861.92746902 -861.81428983
-193.16889479 0.00650038 -0.00000008
-1276.30761624 -861.92746857 861.81429082
-193.17584859 -0.02457904 0.00000000
-1276.12053965 861.59922389 -861.65460775

This shouldn’t be the case for a minimized configuration, right? Shouldn’t
the elements of the Hessian should be positive at a minimum? The matrix’s
eigenvalues are negative as well (using this tool
<https://github.com/xfanak/lammps-dynamical-matrix/tree/main>). The first 8
are:

8562.748779308313
7914.9129841273425
6797.395660302146
-7513.893638723845
6003.365898566742
-6962.005429797687
-5894.101569206084
-5543.162808769083

I have tried the command on other systems as well, and there have been
negative values in every case. I’m not really sure what to make of this.