# [gmx-users] Re: what is sigma in gromacs? the radius of a sphere or the diameter of a sphere?

grita cemilyigit at arcor.de
Thu Aug 15 15:26:05 CEST 2013

Hi Justin,

yes, the LJ potential is zero when the two spheres are at a separation of 5
angstrom.

So, I can be sure, that in this case the \sigma in

c^6 = 4 \epsilon \sigma^6
c^12 = 4 \epsilon \sigma^12

are the seperation or 'my interaction diameter / interaction distance'???

I've asked for this, because in other force fields there are different
definitions of sigma.

Sorry for the confusion.

In short, the sigma is the separation or the 'diameter'. True or False ???

Best, grita

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