Talk to Aidan about this, he has done it, using a new version

of fix npt that does allow for non-orthog fluctuations.

Steve

Talk to Aidan about this, he has done it, using a new version

of fix npt that does allow for non-orthog fluctuations.

Steve

I have a beta version of fix_npt that allows triclinic cell fluctuations.

These fluctuations can be used to sample the full compliance tensor.

Inverting that tensor gives the full elastic constant tensor, at a specified

fixed temperature and pressure. However, convergence of the sample average

to true expectation value is rather slow, as has been noted by others.

LAMMPS fix deform can generate finite shear strains, which should give a

good estimate of C44, provided the strain is sufficiently small. I have only

used this method for zero-temperature elastic constants. It sounds like you

are using fix deform with the erate style and fix npt. Both of these seem

wrong to me. Instead, you should use fix nvt, and fix deform mode in which

the final strain is specified, not the rate e.g.

# uxy Perturbation

variable dneg equal -\{up\}\*{ylen}

# Negative deformation

displace_box all xy delta \{dneg\} units box
:
\# Compute elastic constant from pressure tensor
variable C66neg equal \({pxyneg}-\{pxy0\}\)/\({up})*${pfac1}