[lammps-users] A physical problem in liquid crystal simulation

Dear all,

When liquid crystal bulks experience heating or cooling processes, some phase transition will happened such as crystal- smectic- nematic- isotropic. If we reproduce the process by NPT simulation with the change in temperature a problem comes. It is the control method of pressure.

For phases with no translational order (isotropic and nematic), volume changes were made isotropically that is to say using fix xyz. This scheme is certainly not appropriate for phases with translational order (smectic or solid) because the system has to fit an ordered structure into a box of fixed shape and this may result in unbalanced strain on the system. So we need to use fix xy for smectic phase. Similarly using fix aniso for crystal.

While during the simulation, we can not get the exact step or temperature point to change the control method of pressure. According to my simulation data now, I found that a single layer bulk will be gotten at high temperature if we use fix aniso while a common bulk will be gotten at high temperature if we use fix xyz. At the same time using fix xyz at low temperature, the pressure of z will different from that of x or y. As a conclusion, the control method of pressure will really influence the phase transition of liquid crystal simulation both in theory and in practice. But what is the origination of the relationship? Whether the control method will apply some extra field to the system? If yes, what is the extral field? Is it reasonless to get the simulated phase graph of liquid crystal by adjusting the pressure control method?

Thanks .Any comment will be appreciated!!