Thermal fluctuation while heating of an alloy

I am heating an Al-Si-Mg alloy using npt ensemble from 300 K to 1000 K. For certain composition of the alloy, the temperature fluctuation above 650 K becomes intolerable (~ 0 K - 1800 K). What is the source of such noise? Can you kindly suggest me how to control the temperature? [the input script for LAMMPS simulation is given below]

units metal
atom_style atomic
dimension 3
boundary p p p
pair_style meam
pair_coeff * * library.meam AlS SiS MgS AlSiMg.meam AlS SiS MgS
timestep 0.0002
thermo 1000
dump 1 all xyz 1000 a*.xyz
velocity all create 300 82557 dist gaussian
compute msd all msd com yes
compute 2 all pe/atom
compute 3 all entropy/atom 0.25 5. local yes
dump 2 all custom 50000 x y z c_2 c_3

compute myRDF all rdf 50 1 1 1 2 1 3
fix 3 all vector 1000 c_msd[4]
fix 4 all ave/time 100 1 1000 c_myRDF[*] file tmp1.rdf mode vector
thermo_style custom step temp vol press density ke pe etotal enthalpy c_msd[4]
restart 10000 data.restart

fix 1 all nvt temp 300 300 0.005 drag 0.01
run 10000

unfix 1
fix 1 all npt temp 300 1000 0.001 iso 0 0 2
run 200000

unfix 1
fix 1 all nvt temp 1000 1000 0.005 drag 0.01
run 20000
unfix 1
fix 1 all npt temp 1000 853 .005 iso 0 0 2
run 735000

unfix 1
fix 1 all nvt temp 853 853 0.005 drag 0.01
run 1000000

Hi @Imukherjee.

Please, have a careful look at the manual pages of the fix nvt and units commands.

There is a detailed note on the effect of the Tdamp parameter on temperature convergence and the value you use are way out of range of standard rule-of-thumb values compared to your timestep. Please also see the section regarding the tchain parameter for solids. As a side note, the use of the velocity all create command without a velocity all zero linear is very likely to give you a non zero total linear momentum which can also affect thermal/pressure equilibration.

Your issues can stem from other causes in your systems but these are already independent problems to address. I highly doubt your successive NVT/NPT* are actually done on equilibrated systems what so ever. But without more information concerning your output, this last statement is just a guess on my part.

* Please note, brutal ensemble switching between NPT and NVT are not a good way to proceed. Please see the relevant literature concerning such equilibration procedures.

There always are (local) fluctuations in finite systems. How large the global fluctuations are depend on multiple factors: choice of force field, timestep, initial geometry, system size.

The most straightforward way to reduce fluctuations is to increase the system size.