I want to carry out simulations in LAMMPS for a metallic system.

I am confused as to how to choose the reduced time step.

The unit of time, tau, is tau=sqrt(mass*sigma*sigma/epsilon).

If I want a time-step of 1 femto-second, I will get

different values of the reduced time-step depending on different

values of tau.

For, example, if I choose mass=1 amu, sigma =1 Angstrom

and epsilon =1 ev, I get a reduced time-step 0.1 for 1 femtosecond.

However, if I choose the actual mass of the metal (copper), i.e

63.546 amu, and if I chose sigma to be the value at which the potential

crosses the zero axis, i.e. about 2 Angstrom, and epsilon to be

about 0.1ev, then I get a much lower value for the reduced time-step,

which corresponds to 1 femtosecond.

Is there any rule on how to choose the reduced time-step.

Best Regards,

Ronald