Question about a bad dynamics in He-ion implantation MD simulation


I'm a new learner of MD simulation, and recently I want to simulate a He-ion implantation process to bulk silicon substrates, and obtain the ion's penetration depth.

I have a question about one LAMMPS error message

>ERROR: LOST atoms: oritinal 176081 current 1760400(thermo.cpp:385)

I can't understand why this simulation create a bad dynamics.
I want know how to solve this problem.
My MD simulation is based on the microcanonical, as described below

---------------My LAMMPS input file---------------------

units metal
boundary p p f
atom_style atomic
atom_modify map array

neighbor 1.0 bin
neigh_modify every 1 delay 1 check yes

lattice diamond 5.43
region simbox block -10 10 -10 10 -550 5
region target block -10 10 -10 10 -550 0

create_box 2 simbox
create_atoms 1 region target
mass 1 28.0
group group_si type 1

variable hex equal 1.3575
variable hey equal 2.715
variable hez equal 5.0

print "helium ion initial position is (\{hex\},{hey},${hez})"

create_atoms 2 single \{hex\} {hey} ${hez} units box
mass 2 4.0
group group_he type 2

pair_style hybrid sw lj/cut 10.0

pair_coeff * * sw Si.sw Si Si
pair_coeff 1 2 lj/cut 1.495e-3 2.33

#print "debug 0001"

timestep 0.00025

variable He_energy equal 5.0
variable He_weight equal "4.0/(6.02*10^26)" #Kg
variable He_ene equal 0.16e-18*v_He_energy*1000

variable ww equal "sqrt(2.0*v_He_ene/v_He_weight)" #[m/s]
variable a equal "-v_ww*0.01" #[Angstrome/picosec]
variable af equal "v_a * 0.001"

fix 1 group_si temp/rescale 1 300 300 0.5 1.0
fix 2 all nve

velocity group_si create 300.0 4234929 dist gaussian units box
velocity group_he set 0.0 0.0 $a units box

thermo 1
thermo_style custom step etotal temp press lx ly lz vol
#thermo_modify lnost ignore

dump dump_custom group_he custom 1 dump_all.atom id x y z vx vy vz fx fy fz
dump test_force group_si custom 1 dump_test.atom id x y z vx vy vz fx fy fz

run 10000
---------------------end of LAMMPS input file---------------------------

and result ,

From doc/Section_errors.html

{Lost atoms: original %ld current %ld}
Lost atoms are checked for each time thermo output is done. See the
thermo_modify lost command for options. Lost atoms usually indicate
bad dynamics, e.g. atoms have been blown far out of the simulation
box, or moved futher than one processor's sub-domain away before

So you have to figure out which atoms are lost and where they went.
No one else is likely to do that for you.