# Treating Lennard-Jones point particle as Gay-Berne sphere

Dear LAMMPS user,

I want to simulate a molecule made of an ellipsoid and a point particle. But such a molecule cannot be simulated in NPT ensemble. So instead I was wondering if I can treat the point particle as ellipsoid with equal diameters (sphere)? Please let me know.

Also I was checking if the forces would be same in that case. So I ran two simulations:

1. One ellipsoid (type 1) and one point particle (type 2). The pairs 1-1, 1-2, and 2-2 have GayBerne, GayBerne, and LJ interactions respectively.

The parameters are as below

pair_coeff 1 1 gayberne 1.0 3.5 0.5 1.5 0.5 0.5 1.5 0.5

pair_coeff 1 2 gayberne 1.0 4.1 0.0 0.0 0.0 1.19 1.19 1.19

pair_coeff 2 2 lj/cut 1.19 4.7

The coordinates and force at step 0 are as below

ITEM: ATOMS id type x y z fx fy fz 1 1 -4.41787 -2.47487 -2.47487 -2.40087 0.751521 0

2 2 -0.531874 -2.47487 -2.47487 2.40087 -1.91755 0

3 1 -4.41787 2.47487 -2.47487 -2.40087 -0.751521 0

4 2 -0.531874 2.47487 -2.47487 2.40087 1.91755 0

1. Both particles are ellipsoids. The asymmetric ellipsoid particle (type 1) is same as in the simulation above and the point particle (type 2) is treated as a symmetric ellipsoid (sphere). All the pairs 1-1, 1-2, and 2-2 have GayBerne interactions.

The parameters are as below

pair_coeff 1 1 gayberne 1.0 3.5 0.5 1.5 0.5 0.5 1.5 0.5

pair_coeff 1 2 gayberne 1.0 4.1 0.0 0.0 0.0 1.19 1.19 1.19

pair_coeff 2 2 gayberne 1.0 4.7 1.19 1.19 1.19 1.19 1.19 1.19

The coordinates and forces at step 0 are as below

ITEM: ATOMS id type x y z fx fy fz
1 1 -4.41787 -2.47487 -2.47487 -2.22936 0.97718 0

2 2 -0.531874 -2.47487 -2.47487 2.22936 -1.37674 0

3 1 -4.41787 2.47487 -2.47487 -2.22936 -0.97718 0

4 2 -0.531874 2.47487 -2.47487 2.22936 1.37674 0

However, for the same configuration, the forces are different in the two simulations.
Please let me know what might be wrong. Thank you!