Dear Lammps community,

I want to calculate the orientational order of a 2d system with compute hexorder/atom command. The number of neighbors to be accounted in the calculations is controled by

the parameter nnn. As it is written in the manual if I set nnn NULL then all neighbors up to the cut-off distance are taken into account. (The optional keyword *nnn* defines the number of nearest neighbors used to calculate *qn*. The default value is 6. If the value is NULL, then all neighbors up to the distance cutoff are used.)

As a test case I take a triangular lattice with the lattice spacing about 1.0. If I set the nnn =6 then I obtain correct values of the order parameter:

compute 1 all hexorder/atom cutoff 1.1 nnn 6

ITEM: TIMESTEP

0

ITEM: NUMBER OF ATOMS

20000

ITEM: BOX BOUNDS pp pp pp

0.0000000000000000e+00 1.0745699318235420e+02

0.0000000000000000e+00 1.8612097182041992e+02

-1.0745699318235420e-01 1.0745699318235420e-01

ITEM: ATOMS id c_1[1] c_1[2]

1 1 1.11022e-16

2 1 0

3 1 -1.85037e-17

4 1 0

5 1 0

6 1 0

…

However, if I set nnn NULL the order parameter is not calculated:

compute 1 all hexorder/atom cutoff 1.1 nnn NULL

ITEM: TIMESTEP

0

ITEM: NUMBER OF ATOMS

20000

ITEM: BOX BOUNDS pp pp pp

0.0000000000000000e+00 1.0745699318235420e+02

0.0000000000000000e+00 1.8612097182041992e+02

-1.0745699318235420e-01 1.0745699318235420e-01

ITEM: ATOMS id c_1[1] c_1[2]

1 inf nan

2 inf nan

3 inf nan

4 inf nan

5 inf nan

6 inf nan

Does anyone know what is the problem?

Any help is appreciated.

Sincerely,

Boris