Pcoupltype: isotropic and anisotropic

User discussions
1

GROMACS version: 2018
GROMACS modification: No

Dear all,

I wonder if the two barostat types of isotropic and anisotropic used as below, are practically identical and would give out similar results? Of course by considering the fact that all off-diagonal elements of compressibility are set to zero.

%----------
Pcoupltype = isotropic
tau-p = 2.0
compressibility = 4.46e-5
ref-p = 1.0

%----------
Pcoupltype = anisotropic
tau-p = 2.0
compressibility = 4.46e-5 4.46e-5 4.46e-5 0.0 0.0 0.0
ref-p = 1.0 1.0 1.0 0.0 0.0 0.0 ;( or maybe 1.0 1.0 1.0 1.0 1.0 1.0)

Sincerely,
Salman

2

You will eventually see a drift in the box vectors with anistropic coupling that will lead to changes in the box shape.

3

Thanks for the response.

With that, do you mean, having the off-diagonal elements to be zero does not work well in the long term?

I wonder if I am right that:
In isotropic coupling, only the total pressure is matter to be 1.0 bar so that the projected pressures like P_xx, P_yy, P_zz, P_xy, … are dependent to each others to make the total pressure to be 1.0 bar. Whereas in anisotropic coupling, all the projected pressures are independently 1.0 bar in addition to the total pressure?

4

In the anisotropic scheme, each vector is allowed to oscillate independently whereas the same scaling factor is uniformly applied in all dimensions with isotropic coupling. With anisotropic coupling, I would certainly expect drift. If your medium is homogenous, use isotropic coupling.

5

Dear Jalemkul,

With your answer to S.Z about the differences between isotropic and anisotropic conditions, I wonder, can we use isotropic pressure coupling to a homogenous system even if our box has the same y and z length but different x length?