# Polyhedron elastic collision

Dear LAMMPS users and developers,
While trying to run body/polyhedron examples,
I’ve encountered the following apparently strange behavior.
Oblique collision of two absolutely elastic cubes ( only k_n!=0 ) run with NVE results in considerable jumps of the total energy, either up or down.
I am on Ubuntu 22.04.4 LTS, LAMMPS (2 Aug 2023 - Update 2)
Running
`lmp -in in.cubes`
yields

``````   Step         KinEng         PotEng         TotEng
0   0.25           0              0.25
1000   0.15392932     0              0.15392932
2000   0.15392932     0              0.15392932
3000   0.22513268     0              0.22513268
4000   0.19898377     0              0.19898377
5000   0.2257268      0              0.2257268
6000   0.2257268      0              0.2257268
7000   0.2257268      0              0.2257268
8000   0.2257268      0              0.2257268
9000   0.2418996      0              0.2418996
10000   0.24473686     0              0.24473686
``````

Isn’t it supposed to remain constant for elastic collisions ?
Is the rotational energy taken into account in either kinetic (ke) or total energy (etotal) ?

in.cubes :

``````units       lj

atom_style  body rounded/polyhedron 8 8

variable cut_inner  equal 1
variable k_n        equal 100
variable k_na       equal 0
variable c_n        equal 0
variable c_t        equal 0
variable mu         equal 0
variable delta_ua   equal 1

pair_style body/rounded/polyhedron \${c_n} \${c_t} \${mu} \${delta_ua} \${cut_inner}
pair_coeff * * \${k_n} \${k_na}

comm_modify vel yes

neighbor     0.5 bin
neigh_modify every 1 delay 0 check yes

timestep     0.01

fix          1 all nve/body

thermo_style custom step ke pe etotal
thermo       1000

run          10000
``````

data.cubes :

``````LAMMPS data two cubes
2 atoms
2 bodies
1 atom types
0 15 xlo xhi
0 15 ylo yhi
-15 15 zlo zhi

Atoms

1 1 1 1 4 5 0
2 1 1 1 7 6 0

Velocities

1  0 0 0 0 0 0
2 -1.0 0 0 0 0 0

Bodies

1 3 79
8 12 6
0.667 0.667 0.667 0 0 0
1 1 1
1 -1 1
-1 -1 1
-1 1 1
1 1 -1
1 -1 -1
-1 -1 -1
-1 1 -1
0 1
1 2
2 3
3 0
4 5
5 6
6 7
7 4
0 4
1 5
2 6
3 7
0 1 2 3
4 5 6 7
0 1 5 4
1 2 6 5
2 3 7 6
3 0 4 7
0.5
2 3 79
8 12 6
0.667 0.667 0.667 0 0 0
1 1 1
1 -1 1
-1 -1 1
-1 1 1
1 1 -1
1 -1 -1
-1 -1 -1
-1 1 -1
0 1
1 2
2 3
3 0
4 5
5 6
6 7
7 4
0 4
1 5
2 6
3 7
0 1 2 3
4 5 6 7
0 1 5 4
1 2 6 5
2 3 7 6
3 0 4 7
0.5
``````