# lj units

Hello All,

I’m a little confused by the lj units. If I have multiple mass, sigma and epsilon values in my simulation which one is the base value to use to calculate the non-unitless values? Tau, for example, uses all three of these values…I don’t have multiple time scales just because I have two different mass particles?

Also, to see if I have my head wrapped around this correctly…If I have an epsilon value of 1 then a LJ temperature of 1 is essentially giving the particle sufficient energy to fully dissociate from the LJ pair? Almost an escape velocity?

Hello All,

I’m a little confused by the lj units. If I have multiple mass, sigma and epsilon values in my simulation which one is the base value to use to calculate the non-unitless values?

You can have only one reference epsilon, sigma, and mass. Everything else has to be expressed in relative terms compared to that reference. Those reference values then also define your time scale. The reference can still be anything you like it to be.

Tau, for example, uses all three of these values…I don’t have multiple time scales just because I have two different mass particles?

No, you don’t.

Axel, lets say I have two particles with sigma equal 1 and 2. How does one choose which is the reference sigma? In this case I could say 2 is the reference sigma and 1 is half, or I could say 1 is the reference and 2 is twice the reference.

Axel, lets say I have two particles with sigma equal 1 and 2. How does one choose which is the reference sigma? In this case I could say 2 is the reference sigma and 1 is half, or I could say 1 is the reference and 2 is twice the reference.

The latter.

A value of 1 means that this sigma is identical to the reference. Same for epsilon and mass.

The reference can still be anything you like it to be in real units. That is what reduced units are all about.

Axel

Also, check out Appendix B in Allen & Tildesley for a discussion
of LJ units.

Steve