Some code clean-up, added safety check in post_force_velocity.

dimension 2

boundary p p p

variable L equal 20

region total block -$L $L -$L $L -0.5 0.5

lattice hex 0.3

create_box 2 total

create_atoms 1 box

# Set random fraction to passive:

set type 1 type/fraction 2 0.5 1337

# Purely repulsive particles:

variable rc equal "2^(1.0/6.0)"

pair_style lj/cut ${rc}

pair_coeff * * 1.0 1.0

pair_modify shift yes

mass * 1.0

fix step all nve

fix temp all langevin 1.0 1.0 1.0 13

fix twod all enforce2d

neighbor 0.6 bin

dump traj all custom 250 2d_active.dump.bin id type x y z

thermo_style custom time step pe ke etotal temp

thermo 1000

run 25000

fix active all propel/self velocity 1.0

# With active force there is more motion so increase bin size:

neighbor 1.0 bin

run 25000

dimension 2

boundary p p p

variable L equal 20

region total block -$L $L -$L $L -0.5 0.5

lattice hex 0.3

create_box 2 total

create_atoms 1 box

# Set random fraction to passive:

set type 1 type/fraction 2 0.5 1337

# Purely repulsive particles:

variable rc equal "2^(1.0/6.0)"

pair_style lj/cut ${rc}

pair_coeff * * 1.0 1.0

pair_modify shift yes

mass * 1.0

fix step all nve

fix twod all enforce2d

neighbor 0.6 bin

dump traj all custom 250 2d_active.dump.bin id type x y z

thermo_style custom time step pe ke etotal temp

thermo 1000

run 25000

fix active all propel/self velocity 1.0

fix fric all viscous 1.0

# With active force there is more motion so increase bin size:

neighbor 1.0 bin

run 25000

dimension 3

boundary p p p

atom_style ellipsoid

variable L equal 20

region total block -$L $L -$L $L -$L $L

lattice sc 0.1

create_box 2 total

create_atoms 1 box

# Set random fraction to passive:

set type 1 type/fraction 2 0.5 1337

# Purely repulsive particles:

variable rc equal "2^(1.0/6.0)"

pair_style lj/cut ${rc}

pair_coeff * * 1.0 1.0

pair_modify shift yes

# mass * 1.0

set type * density 1.0

set type * shape 1.0 1.0 1.0

set type * quat 0 0 1 0

fix step all nve

fix temp all langevin 1.0 1.0 1.0 13

neighbor 0.6 bin

dump traj all custom 100 3d_active.dump.bin id type x y z fx fy fz

thermo_style custom time step pe ke etotal temp

thermo 100

run 5000

fix active all propel/self quaternion 1.0

# With active force there is more motion so increase bin size:

neighbor 1.0 bin

run 5000

      const double *vi = v[i];

      double f_act[3] = { vi[0], vi[1], vi[2] };

      double nv2 = vi[0]*vi[0] + vi[1]*vi[1] + vi[2]*vi[2];

	    double fnorm = magnitude / sqrt(nv2);

      double fnorm = 0.0;

      constexpr const double TOL = 1e-14;

      if (nv2 > TOL) {

        // Without this check you can run into numerical issues

        // because fnorm will blow up.

        fnorm = magnitude / sqrt(nv2);

      }

      if (debug_out && comm->me == 0) {

        // Magical reference particle: