The temperature keeps rising with NVE, pair_style dpd, to simulate an amphiphilic polymeric system.

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1

Dear all,
I am using lammps-14May16 to simulate the self-assemble of an amphiphilic polymer in aqueous solution. I want to use pair_style DPD to thermostat the system and the NVE ensemble to intergrate the system. However, the output temperature keeps increasing during the simulation process (see below). I have worked on this problem for several days but failed to find the solution. I need your help. Thank you very much.
Please find attached the ‘in’ and ‘data’ file and see if I can make modifications to sole the problem. Your efforts are greatly appreciated.

Step Temp Press Density Volume TotEng E_bond KinEng

0 1 -3.0649994 0.60223104 15625000 684.75832 683.25821 1.4999857
100 296.8033 0.3046252 0.60223104 15625000 817.30913 372.10585 445.2007
200 347.08839 1.1499406 0.60223104 15625000 778.84213 258.21002 520.6276
300 295.39644 0.063997436 0.60223104 15625000 866.13804 423.04076 443.09043
400 392.42393 1.1196085 0.60223104 15625000 921.43854 332.80002 588.63027
500 424.5713 1.1219761 0.60223104 15625000 1016.4616 379.60204 636.85086
600 387.93095 0.35429664 0.60223104 15625000 1078.2246 496.32579 581.89086
700 489.8567 1.4549567 0.60223104 15625000 1137.7066 402.91989 734.77803
800 449.20857 0.98352717 0.60223104 15625000 1120.8352 447.02047 673.80641
900 427.89193 0.45870376 0.60223104 15625000 1174.331 532.49057 641.83176
1000 501.01184 1.1540981 0.60223104 15625000 1237.7495 486.22921 751.51056
…

best regards
Wang Zhikun
China University of Petroleum (East China)
Email: wzhikun0806@…127…

in.system_nve (1.34 KB)

2

Have you tried a smaller timestep? Do you mean you are using

pair_style dpd/tstat to thermostat?

Steve

3

Hi Steve,
I have tried a smaller timestep 0.01 or 0.001, yet the problem remains to be unsolved. I am using pair_style dpd rather than dpd/tstat.

Zhikun Wang

4

Well then you are not thermostatting at all, as your original email

indicated.

Steve

5

Well then you are not thermostatting at all, as your original email
indicated.

sorry to correct you, steve, but pair style dpd *does* include a
thermostat, the first argument in the pair style command is the target
temperature.

axel

6

good point,I forgot about the temp setting in pair_style dpd.
If you run long enough and your system is equilibrated,

I think it should come back to the desired temperature. I think

many people run DPD in reduced units (LJ units in LAMMPS),

so possibly you have parameterized something else inappropriately

for real units. I also note that NVE for standard DPD is known

to not conserve energy to high fidelity. Which is why people use

dpd/tstat, or there are other modified integrators (like those in the USER-DPD)

package which are better at conserving energy. But a 2x increase

in temperature in 1000 steps indicates something else is likely wrong with

your model.

Steve