This is part of the multicolvar module

Calculate number of pairs of atoms/molecules that are "linked"

In its simplest guise this coordinate calculates a coordination number. Each pair of atoms is assumed "linked" if they are within some cutoff of each other. In more complex applications each entity is a vector and this quantity measures whether pairs of vectors are (a) within a certain cutoff and (b) if the two vectors have similar orientations. The vectors on individual atoms could be Steinhardt parameters (see Q3, Q4 and Q6) or they could describe some internal vector in a molecule.

Compulsory keywords

DATA	the labels of the action that calculates the multicolvars we are interested in
NN	( default=6 ) The n parameter of the switching function
MM	( default=12 ) The m parameter of the switching function
D_0	( default=0.0 ) The d_0 parameter of the switching function
R_0	The r_0 parameter of the switching function

Options

NOPBC

( default=off ) ignore the periodic boundary conditions when calculating distances

SERIAL

( default=off ) do the calculation in serial. Do not parallelize

LOWMEM

( default=off ) lower the memory requirements

TOL

this keyword can be used to speed up your calculation. When accumulating sums in which the individual terms are numbers inbetween zero and one it is assumed that terms less than a certain tolerance make only a small contribution to the sum. They can thus be safely ignored as can the the derivatives wrt these small quantities.

SWITCH

This keyword is used if you want to employ an alternative to the continuous swiching function defined above. The following provides information on the switchingfunction that are available. When this keyword is present you no longer need the NN, MM, D_0 and R_0 keywords.

Examples

The following calculates how many bonds there are in a system containing 64 atoms and outputs this quantity to a file.

DENSITY SPECIES=1-64 LABEL=d1
NLINKS ARG=d1 SWITCH={RATIONAL D_0=1.3 R_0=0.2} LABEL=dd
PRINT ARG=dd FILE=colvar

The following calculates how many pairs of neighbouring atoms in a system containg 64 atoms have similar dispositions for the atoms in their coordination sphere. This calculation uses the dot product of the Q6 vectors on adjacent atoms to measure whether or not two atoms have the same ``orientation"

Q6 SPECIES=1-64 SWITCH={RATIONAL D_0=1.3 R_0=0.2} LABEL=q6
NLINKS ARG=q6 SWITCH={RATIONAL D_0=1.3 R_0=0.2} LABEL=dd
PRINT ARG=dd FILE=colvar