# How can I know if 3D aperiodic systems are not interacting with each other using Quantum ESPRESSO?

I wanted to observe the Projected Density of States of InP system passivated with Hydride ions. For that, the following input file was created.

&CONTROL
calculation='scf',
outdir='./outdir',
prefix='InPH',
pseudo_dir='.',
verbosity='low',
tprnfor=.true.,
tstress=.true.,
/
&SYSTEM
ibrav = 0
A =   28.95873
nat = 38
ntyp = 3
ecutwfc=50,
ecutrho=400,
input_dft='pbe',
occupations='smearing',
starting_charge(1) = +3.0
starting_charge(2) = -3.0
starting_charge(3) = -1.0
smearing='mv',
degauss=0.005d0,

/

&ELECTRONS
conv_thr=1d-07,
mixing_beta=0.7d0,
/

CELL_PARAMETERS {alat}
1.000000000000000   0.000000000000000   0.000000000000000
0.000000000000000   1.000000000000000   0.000000000000000
0.000000000000000   0.000000000000000   1.000000000000000
ATOMIC_SPECIES
In  114.81800   In.pbe-dn-kjpaw_psl.1.0.0.UPF
P   30.97300   P.pbe-n-kjpaw_psl.1.0.0.UPF
H    1.00750   H.pbe-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {crystal}
In   0.301777000000000   0.515510000000000   0.403224000000000
In   0.489752000000000   0.303558000000000   0.415264000000000
In   0.503499000000000   0.505059000000000   0.408072000000000
In   0.501909000000000   0.513929000000000   0.598334000000000
In   0.296909000000000   0.419582000000000   0.514790000000000
In   0.492873000000000   0.398390000000000   0.315705000000000
In   0.497050000000000   0.408959000000000   0.504514000000000
In   0.510066000000000   0.608991000000000   0.499706000000000
In   0.390717000000000   0.315763000000000   0.518452000000000
In   0.401476000000000   0.501161000000000   0.310056000000000
In   0.402099000000000   0.515299000000000   0.498794000000000
In   0.602463000000000   0.505451000000000   0.505306000000000
In   0.393310000000000   0.407603000000000   0.415739000000000
In   0.399042000000000   0.422615000000000   0.605304000000000
In   0.406187000000000   0.607978000000000   0.395822000000000
In   0.594811000000000   0.396109000000000   0.407363000000000
P   0.448966000000000   0.459054000000000   0.454312000000000
P   0.440175000000000   0.354263000000000   0.367464000000000
P   0.445543000000000   0.367260000000000   0.559101000000000
P   0.455293000000000   0.551606000000000   0.354347000000000
P   0.454803000000000   0.563518000000000   0.546515000000000
P   0.348592000000000   0.457212000000000   0.361770000000000
P   0.351886000000000   0.472249000000000   0.553642000000000
P   0.546899000000000   0.448683000000000   0.359923000000000
P   0.548477000000000   0.458559000000000   0.552198000000000
P   0.341560000000000   0.363924000000000   0.467769000000000
P   0.354314000000000   0.565300000000000   0.448689000000000
P   0.543486000000000   0.352663000000000   0.460158000000000
P   0.556644000000000   0.554867000000000   0.452761000000000
H   0.642987000000000   0.469927000000000   0.474034000000000
H   0.481225000000000   0.651348000000000   0.464263000000000
H   0.255512000000000   0.450777000000000   0.480210000000000
H   0.526102000000000   0.358078000000000   0.281589000000000
H   0.522877000000000   0.266367000000000   0.377674000000000
H   0.360938000000000   0.390592000000000   0.642932000000000
H   0.271709000000000   0.550732000000000   0.361406000000000
H   0.371546000000000   0.639032000000000   0.354239000000000
H   0.367327000000000   0.536759000000000   0.271849000000000

K_POINTS {automatic}
1 1 1  0 0 0


Here the quantum dot has 13 P$$^{-3}$$ ions, 16 In$$^{+3}$$ ions and 9 H$$^{-}$$. Hence the starting charge.

How can I know if the systems are not interacting with each other? The diameter of the system is 8 angstrom and the unit cell is cubic with a cell parameter of 28 angstroms

NOTE: The convergence of observables w.r.t variation in Cell parameters (as Tristan suggested) is one method. But if there are other more quantifiable methods like calculating the electron density in between two particles in adjacent cells, it would be nice to know :)