## Bug #14

### Rahman Parrinello barostat not accurate for the anisotropic case.

**Description**

The implementation of the Rahman Parrinello barostat does not match the one

published in M. Parrinello, A. Rahaman J. Appl. Phys vol 52 n 12 page 7182

For the case where only isotropic pressure is applied, the implementation is

correct. However, when there is an non isotropic stress tensor, Gromacs uses an

incorrect generalization that consists in replacing the pressure by the stress

tensor in the formula:

W box'' = (pressure_observed - pressure) box^{-1} * volume

This seems a natural generalization. In fact first paper anticipating the Rahman

Parrinello thermostat (PRL vol 45, n 14, pg 1196) suggested this extension. But

the paper (earlier mentioned) that developed the method gives a different formula.

A posible solution is to support isotropic pressure only, even when allowing box

deformations. This is done by DL_POLY. In fact, the review article of Nose and

Klein (Molecular Physics, vol 50, n 5 pg 1055) cited by Gromacs does not mention

how to implement a barostat to a stress tensor. Furthermore according to the

paper by Rahman and Parrinello, if a external stress tensor is applied, both the

hydrostatic pressure and the tensor have to be specified. So ref_p for the

anisotropic case would need 7 components, one for the isotropic pressure and 6

for the external stress tensor.

### History

#### #1 Updated by Erik Lindahl about 14 years ago

Hi Ramon,

Thanks for the report. This will *probably* not be fixed right now, since Michael Shirts is working on a

complete overhaul of the Parinello-Rahman (and Anderson) pressure scaling.

The problem is that to conserve enthalphy we cannot reformulate the equation of motion from scaled to

normal coordinates like we do now. It also requires velocity verlet integration, which in turn means we

need the velocity constraint correction step in Lincs and Settle (Rattle). Too many things depending on

each other...

Nevertheless, I'll try to have a look at the article.

#### #2 Updated by David van der Spoel over 12 years ago

I'm closing this bug, it has now been listed as a development target at the gromacs wiki: