compute coord/atom command

Syntax

compute ID group-ID coord/atom cstyle args ...
  • ID, group-ID are documented in compute command

  • coord/atom = style name of this compute command

  • cstyle = cutoff or orientorder

    cutoff args = cutoff typeN
      cutoff = distance within which to count coordination neighbors (distance units)
      typeN = atom type for Nth coordination count (see asterisk form below)
    orientorder args = orientorderID threshold
      orientorderID = ID of an orientorder/atom compute
      threshold = minimum value of the product of two "connected" atoms
    

Examples

compute 1 all coord/atom cutoff 2.0
compute 1 all coord/atom cutoff 6.0 1 2
compute 1 all coord/atom cutoff 6.0 2*4 5*8 *
compute 1 all coord/atom orientorder 2 0.5

Description

This compute performs calculations between neighboring atoms to determine a coordination value. The specific calculation and the meaning of the resulting value depend on the cstyle keyword used.

The cutoff cstyle calculates one or more traditional coordination numbers for each atom. A coordination number is defined as the number of neighbor atoms with specified atom type(s) that are within the specified cutoff distance from the central atom. Atoms not in the specified group are included in the coordination number tally.

The typeN keywords allow specification of which atom types contribute to each coordination number. One coordination number is computed for each of the typeN keywords listed. If no typeN keywords are listed, a single coordination number is calculated, which includes atoms of all types (same as the “*” format, see below).

The typeN keywords can be specified in one of two ways. An explicit numeric value can be used, as in the 2nd example above. Or a wild-card asterisk can be used to specify a range of atom types. This takes the form “*” or “*n” or “n*” or “m*n”. If N = the number of atom types, then an asterisk with no numeric values means all types from 1 to N. A leading asterisk means all types from 1 to n (inclusive). A trailing asterisk means all types from n to N (inclusive). A middle asterisk means all types from m to n (inclusive).

The orientorder cstyle calculates the number of “connected” neighbor atoms J around each central atom I. For this cstyle, connected is defined by the orientational order parameter calculated by the compute orientorder/atom command. This cstyle thus allows one to apply the ten Wolde’s criterion to identify crystal-like atoms in a system, as discussed in ten Wolde.

The ID of the previously specified compute orientorder/atom command is specified as orientorderID. The compute must invoke its components option to calculate components of the Ybar_lm vector for each atoms, as described in its documentation. Note that orientorder/atom compute defines its own criteria for identifying neighboring atoms. If the scalar product (Ybar_lm(i),*Ybar_lm(j)*), calculated by the orientorder/atom compute is larger than the specified threshold, then I and J are connected, and the coordination value of I is incremented by one.

For all cstyle settings, all coordination values will be 0.0 for atoms not in the specified compute group.

The neighbor list needed to compute this quantity is constructed each time the calculation is performed (i.e. each time a snapshot of atoms is dumped). Thus it can be inefficient to compute/dump this quantity too frequently.

Note

If you have a bonded system, then the settings of special_bonds command can remove pairwise interactions between atoms in the same bond, angle, or dihedral. This is the default setting for the special_bonds command, and means those pairwise interactions do not appear in the neighbor list. Because this fix uses the neighbor list, it also means those pairs will not be included in the coordination count. One way to get around this, is to write a dump file, and use the rerun command to compute the coordination for snapshots in the dump file. The rerun script can use a special_bonds command that includes all pairs in the neighbor list.

Output info:

For cstyle cutoff, this compute can calculate a per-atom vector or array. If single type1 keyword is specified (or if none are specified), this compute calculates a per-atom vector. If multiple typeN keywords are specified, this compute calculates a per-atom array, with N columns.

For cstyle orientorder, this compute calculates a per-atom vector.

These values can be accessed by any command that uses per-atom values from a compute as input. See Section 6.15 for an overview of LAMMPS output options.

The per-atom vector or array values will be a number >= 0.0, as explained above.

Restrictions

none