# Lennard-jones cut-off

I am using a Lennard-Jones potential in a computer simulation and I was wondering if i should place a cut-off for low distances since the potential blows up quickly making the calculations quite difficult. I am trying to simulate basic molecules ( few atoms ) , calculating bonds stretching , bond angle bending and lennard-jones interactions and trying to minimize the total energy to get an optimal structure prediction .

• Please describe the system you are trying to simulate. What exactly does 'blow up' refer to? If it is uncontrollably high energies, then you probably will not benefit at all from having a low value for the cut-off, since the energy from LJ-potential grows only when the intermolecular distances are small.
– Bdrs
Feb 19, 2017 at 11:19
• Still need more info: Which forcefield are you using? If you are not using any general FF, then please point to the source where you are getting the parameter values. Which simulator are you using, or are you doing the calculations yourself? Which molecules are you trying to simulate? Are you simulating them in a solvent, or in vacuum? Do all of the particles have LJ-sites? Also, please elaborate on the 'blowing up'. If you are using any standard MD software (GROMACS, DL_POLY etc.) you should be able to see which energy term(s) grow unphysically high.
– Bdrs
Feb 19, 2017 at 11:51
• I am building my own simple force field in python, using parameters from : github.com/labello/amberff/blob/master/gaffparams.py . I am doing all the calculations myself .I hope to simulate simple molecules in vacuum like hydrocarbons . I am programming everything from scratch including the optimization method , the Lennard-Jones term keeps getting very large Feb 19, 2017 at 13:10
• LJ potentials should get really large only if distances are small. Maybe that's what you got? Feb 20, 2017 at 12:16

• Check that you have $\sigma$ and $\epsilon$ values inserted in the correct order.