Dear all:

Recently, I calculate the NPT ensemble (tri or aniso). All the parametes are set as the defalut values.

But after 800000 steps. the each length of the box is changed, as shown in attached file.

I have tried to use different Tdamp and Pdamp, but it still occurs.

When I set the pchain = 0, such strange phonomenon is gone.

Thus I’m wondering the meaning of tchain and pchain.

And is there any trick in NPT controlling.

Any suggestion that how to change this situation?

thank you very much.

If you are running NPT, why wouldn't the length of the box

change?

Steve

Dear Steve

The length of the box should be changed in NPT.

But the average should be the same due to the BCC structure.

My result shows that the average length of box are distinct after 800000 steps.

I just want to simulate the BCC structure for 10000000 steps.

Dear Steve

```
The length of the box should be changed in NPT\.
But the average should be the same due to the BCC structure\.
My result shows that the average length of box are distinct after
```

800000 steps.

I just want to simulate the BCC structure for 10000000 steps.

have you visualized your trajectory data?

perhaps the BCC structure is not stable with your choice of potentials?

axel.

If you are saying that you started with a box of

a particular shape (e.g. a cube), then ran NPT and

the box shape changed (no longer a cube), then

that is a function of what NPT option you chose.

That won't happen for "iso" since all 3 box lengths

change together. It can happen for aniso or tri

(which you said you used), b/c they allow each of

the 3 box lenghts to change independently.

Steve