# Controlling lattice anharmonicity

Hi Lammps Users,

I was wondering is there a standard way to control lattice anharmonicity for common forcefields used for dielectric lattices. For example, how can I change lattice anharmonicity in Si? Usually, for Si, SW or Tersoff forcefields are employed. Is there any parameter in the forcefield description, that may tune the anharmonicity? From my understanding, lattice anharmonicity or nonlinearity sometimes characterized by Gruneisen parameter. Thus essentially I am trying to vary Gruneisen parameter for a lattice. One idea I had was to use a class 2 forcefield for Si. I know it is not commonly used for crystalline si, but the motivation is : now I can control the cubic or fourth order terms in bond stretching to make the inter-atomic interactions more harmonic.

Thanks and regards,
Souvik

Hi Lammps Users,

I was wondering is there a standard way to control lattice anharmonicity for
common forcefields used for dielectric lattices. For example, how can I
change lattice anharmonicity in Si? Usually, for Si, SW or Tersoff
forcefields are employed. Is there any parameter in the forcefield
description, that may tune the anharmonicity? From my understanding, lattice
anharmonicity or nonlinearity sometimes characterized by Gruneisen
parameter. Thus essentially I am trying to vary Gruneisen parameter for a
lattice. One idea I had was to use a class 2 forcefield for Si. I know it is
not commonly used for crystalline si, but the motivation is : now I can
control the cubic or fourth order terms in bond stretching to make the
inter-atomic interactions more harmonic.

there is a very fundamental problem with your question: there is
typically not a direct relationship between individual force field
parameters and thermodynamic properties of the resulting system. in
general, all parameters have an impact on all thermodynamic properties
and they are not necessarily linear either. this explains where it is
so difficult to get force field parameters that are a good
representation for all thermodynamic properties. so if you change some
parameters to vary the grueneisen parameter, you will also change
other properties of your system in an uncontrolled fashion. you would
have to rebalance all parameters at the same time to retain the
remaining properties as good as possible.

axel.