Temperature control problem

LAMMPS
1

Dear LAMMPS users

I want to increase the temperature to see how the structure changes. Why the size of the system increases and then decreases. And then make it bigger and smaller? The temperature control code is“fix 2 all npt temp 300.0 900.0 0.1 x 0.0 0.0 0.1 y 0.0 0.0 0.1”Here is the log result:

Step Temp E_pair E_mol TotEng Press Volume
27000 499.97925 -1615698.4 0 -1591030.3 -13269.806 531348.26
27100 500.47147 -1615470.6 0 -1590778.1 -12936.637 531293.51
27200 502.42131 -1615945.9 0 -1591157.3 -12570.599 531248.96
27300 504.05067 -1614990.2 0 -1590121.2 -12432.655 531277.78
27400 504.85971 -1615369 0 -1590460 -12644.787 531480.95
27500 505.72191 -1614068.5 0 -1589117 -12472.496 531468.79
27600 506.6344 -1615039 0 -1590042.5 -12098.366 531362.43
27700 506.51001 -1614721.2 0 -1589730.9 -12468.232 531568.21
27800 507.49505 -1615722.7 0 -1590683.8 -12449.407 531579.35
27900 510.36764 -1615507.4 0 -1590326.7 -12210.849 531493.08
28000 512.83811 -1614282.4 0 -1588979.8 -12760.346 531757.48
28100 511.2948 -1614521.5 0 -1589295.1 -12694.812 531712.03
28200 506.50835 -1615018.8 0 -1590028.5 -12896.786 531770.54
28300 510.86762 -1615405.2 0 -1590199.8 -13133.25 531833.53
28400 517.74152 -1614910.7 0 -1589366.2 -13217.752 531790.72
28500 516.15476 -1614057.8 0 -1588591.5 -13485.382 531796.15
28600 514.40829 -1615667.3 0 -1590287.2 -13007.995 531461.72
28700 516.27071 -1617096.2 0 -1591624.2 -12566.942 531146.66
28800 516.5483 -1614150.5 0 -1588664.9 -12445.116 530958.66
28900 514.6701 -1615230.1 0 -1589837.1 -12602.06 530970.35
29000 514.54098 -1616175.6 0 -1590789 -12989.74 531056.71
29100 517.10228 -1613778.1 0 -1588265.1 -13007.072 530922.08
29200 520.63969 -1616507.1 0 -1590819.6 -12482.801 530555.91
29300 520.68073 -1614568.3 0 -1588878.8 -12376.403 530348.09
29400 520.21206 -1617466.4 0 -1591800 -12430.661 530228.69
29500 522.14465 -1616354.6 0 -1590592.9 -13374.787 530486.5
29600 524.01784 -1617988.3 0 -1592134.1 -12636.812 529987.27
29700 526.34455 -1618162.4 0 -1592193.4 -12074.506 529547.92
29800 528.77706 -1618756.2 0 -1592667.2 -12553.743 529563.24
29900 528.06819 -1619042.1 0 -1592988 -13069.411 529588.58
30000 528.76557 -1619373.1 0 -1593284.7 -11853.381 528822.22
30100 528.69653 -1620236.5 0 -1594151.5 -12443.787 528845.65
30200 529.0729 -1620293.6 0 -1594190 -12619.469 528655.18
30300 530.20478 -1620033.8 0 -1593874.4 -11443.929 527829.45
30400 530.91771 -1620451.1 0 -1594256.5 -12478.679 527988.4
30500 532.3889 -1620965.6 0 -1594698.4 -12315.422 527618.8

2

Is it really the question you want to ask?

3

Thank you for your question. The question I want to ask is: Why doesn’t the volume continue to increase as the temperature rises?

4

Well, without knowing what your system is, it’s impossible to say.

1 Like
5

Without knowing the thermal history of this simulation, the timestep, and the composition, it’s impossible to make any meaningful assessment. Also, we don’t know how long is the run in which the fix is executed, even though it appears to be around 70K steps.

Said so, the increase of volume happens during the first 1500 steps, which is entirely compatible with statistical noise.

6

That is impossible to tell with such limited information.
For example, it cannot be determined whether your system been properly equilibrated before you start your run.

Please also note that your temperature ramp is extremely rapid and thus your system does not have sufficient time to react. You have to keep length and time scales in mind when setting up simulations at the atomic and cannot expect them to behave like bulk systems on “human” time and length scales.