\(\renewcommand{\AA}{\text{Å}}\)

# fix gravity command

Accelerator Variants: gravity/omp, gravity/kk

## Syntax

    fix ID group gravity magnitude style args

- ID, group are documented in fix command

- gravity = style name of this fix command

- magnitude = size of acceleration (force/mass units), magnitude can be a variable (see below)

- style = chute or spherical or gradient or vector

      chute args = angle
        angle = angle in +x away from -z or -y axis in 3d/2d (in degrees)
        angle can be a variable (see below)
      spherical args = phi theta
        phi = azimuthal angle from +x axis (in degrees)
        theta = angle from +z or +y axis in 3d/2d (in degrees)
        phi or theta can be a variable (see below)
      vector args = x y z
        x y z = vector direction to apply the acceleration
        x or y or z can be a variable (see below)

## Examples

    fix 1 all gravity 1.0 chute 24.0
    fix 1 all gravity v_increase chute 24.0
    fix 1 all gravity 1.0 spherical 0.0 -180.0
    fix 1 all gravity 10.0 spherical v_phi v_theta
    fix 1 all gravity 100.0 vector 1 1 0

## Description

Impose an additional acceleration on each particle in the group. This fix is typically used with granular systems to include a “gravity” term acting on the macroscopic particles. More generally, it can represent any kind of driving field, e.g. a pressure gradient inducing a Poiseuille flow in a fluid. Note that this fix operates differently than the fix addforce command. The addforce fix adds the same force to each atom, independent of its mass. This command imparts the same acceleration to each atom (force/mass).

The magnitude of the acceleration is specified in force/mass units. For granular systems (LJ units) this is typically 1.0. See the units command for details.

Style chute is typically used for simulations of chute flow where the specified angle is the chute angle, with flow occurring in the +x direction. For 3d systems, the tilt is away from the z axis; for 2d systems, the tilt is away from the y axis.

Style spherical allows an arbitrary 3d direction to be specified for the acceleration vector. Phi and theta are defined in the usual spherical coordinates. Thus for acceleration acting in the -z direction, theta would be 180.0 (or -180.0). Theta = 90.0 and phi = -90.0 would mean acceleration acts in the -y direction. For 2d systems, phi is ignored and theta is an angle in the xy plane where theta = 0.0 is the y-axis.

Style vector imposes an acceleration in the vector direction given by (x,y,z). Only the direction of the vector is important; it’s length is ignored. For 2d systems, the z component is ignored.

Any of the quantities magnitude, angle, phi, theta, x, y, z which define the gravitational magnitude and direction, can be specified as an equal-style variable. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and its value used to determine the quantity. You should ensure that the variable calculates a result in the appropriate units, e.g. force/mass or degrees.

Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent gravitational field.

Styles with a gpu, intel, kk, omp, or opt suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the Accelerator packages page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Build package page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Accelerator packages page for more instructions on how to use the accelerated styles effectively.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to binary restart files.

The fix_modify energy option is supported by this fix to add the gravitational potential energy of the system to the global potential energy of the system as part of thermodynamic output. The default setting for this fix is fix_modify energy no.

The fix_modify respa option is supported by this fix. This allows to set at which level of the r-RESPA integrator the fix is adding its forces. Default is the outermost level.

This fix computes a global scalar which can be accessed by various output commands. This scalar is the gravitational potential energy of the particles in the defined field, namely mass * (g dot x) for each particles, where x and mass are the particles position and mass, and g is the gravitational field. The scalar value calculated by this fix is “extensive”.

No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

## Restrictions

none

## Related commands

atom_style sphere, fix addforce

## Default

none