dynamical_matrix command¶

Syntax¶

dynamical_matrix group-ID style gamma args keyword value ...

group-ID = ID of group of atoms to displace
style = regular or eskm
gamma = finite different displacement length (distance units)

one or more keyword/arg pairs may be appended

keyword = file or binary
  file name = name of output file for the dynamical matrix
  binary arg = yes or no or gzip

Examples¶

dynamical_matrix 1 regular 0.000001
dynamical_matrix 1 eskm 0.000001
dynamical_matrix 3 regular 0.00004 file dynmat.dat
dynamical_matrix 5 eskm 0.00000001 file dynamical.dat binary yes

Description¶

Calculate the dynamical matrix by finite difference of the selected group,

\[D = \frac{\Phi_{ij}^{\alpha\beta}}{\sqrt{M_i M_j}}\]

where D is the dynamical matrix and \(\Phi\) is the force constant matrix defined by

\[\Phi_{ij}^{\alpha\beta} = \frac{\partial^2 U}{\partial x_{i,\alpha} \partial x_{j,\beta}}\]

The output for the dynamical matrix is printed three elements at a time. The three elements are the three \(\beta\) elements for a respective i/\(\alpha\)/j combination. Each line is printed in order of j increasing first, \(\alpha\) second, and i last.

If the style eskm is selected, the dynamical matrix will be in units of inverse squared femtoseconds. These units will then conveniently leave frequencies in THz.

Restrictions¶

The command collects an array of nine times the number of atoms in a group on every single MPI rank, so the memory requirements can be very significant for large systems.

This command is part of the PHONON package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

Default¶

The default settings are file = “dynmat.dyn”, binary = no