compute property/grid command — LAMMPS documentation

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# compute property/grid command

## Syntax

    compute ID group-ID property/grid Nx Ny Nz input1 input2 ...

- ID, group-ID are documented in compute command

- property/grid = style name of this compute command

- Nx, Ny, Nz = grid size in each dimension

- input1,etc = one or more attributes

      attributes = id, ix, iy, iz, x, y, z, xs, ys, zs, xc, yc, zc, xsc, ysc, zsc
        id = ID of grid cell, x fastest, y next, z slowest
        ix,iy,iz = grid indices in each dimension (1 to N inclusive)
        x,y,z = coords of lower left corner of grid cell
        xs,ys,zs = scaled coords of lower left corner of grid cell (0.0 to 1.0)
        xc,yc,zc = coords of center point of grid cell
        xsc,ysc,zsc = scaled coords of center point of grid cell (0.0 to 1.0)

## Examples

    compute 1 all property/grid id ix iy iz
    compute 1 all property/grid id xc yc zc

## Description

Define a computation that stores the specified attributes of a distributed grid. In LAMMPS, distributed grids are regular 2d or 3d grids which overlay a 2d or 3d simulation domain. Each processor owns the grid cells whose center points lie within its subdomain. See the Howto grid doc page for details of how distributed grids can be defined by various commands and referenced.

This compute stores the specified attributes of grids as per-grid data so they can be accessed by other output commands such as dump grid.

Nx, Ny, and Nz define the size of the grid. For a 2d simulation Nz must be 1. When this compute is used by dump grid, to output per-grid values from other computes of fixes, the grid size specified for this command must be consistent with the grid sizes used by the other commands.

The id attribute stores the grid ID for each grid cell. For a global grid of size Nx by Ny by Nz (in 3d simulations) the grid IDs range from 1 to Nx*Ny*Nz. They are ordered with the X index of the 3d grid varying fastest, then Y, then Z slowest. For 2d grids (in 2d simulations), the grid IDs range from 1 to Nx*Ny, with X varying fastest and Y slowest.

The ix, iy, iz attributes are the indices of a grid cell in each dimension. They range from 1 to Nx inclusive in the X dimension, and similar for Y and Z.

The x, y, z attributes are the coordinates of the lower left corner point of each grid cell.

The xs, ys, zs attributes are also coordinates of the lower left corner point of each grid cell, except in scaled coordinates, where the lower-left corner of the entire simulation box is (0,0,0) and the upper right corner is (1,1,1).

The xc, yc, zc attributes are the coordinates of the center point of each grid cell.

The xsc, ysc, zsc attributes are also coordinates of the center point each grid cell, except in scaled coordinates, where the lower-left corner of the entire simulation box is (0,0,0) and the upper right corner is (1,1,1).

For triclinic simulation boxes, the grid point coordinates for (x,y,z) and (xc,yc,zc) will reflect the triclinic geometry. For (xs,yz,zs) and (xsc,ysc,zsc), the coordinates are the same for orthogonal versus triclinic boxes.

## Output info

This compute calculates a per-grid vector or array depending on the number of input values. The length of the vector or number of array rows (distributed across all processors) is Nx * Ny * Nz. For access by other commands, the name of the single grid produced by this command is “grid”. The name of its per-grid data is “data”.

The (x,y,z) and (xc,yc,zc) coordinates are in distance units.

## Restrictions

For 2d simulations, the attributes which refer to the Z dimension cannot be used.

## Related commands

dump grid

## Default

none