[lammps-users] charmm LJ energy with lammps

LAMMPS LAMMPS Mailing List Mirror
1

Hi

I am trying to use lammps to compute LJ energy as per charmm ff.

The system configuration is as follows:
a ring with 6 atoms on the vertices of a regular hexagon. All 6 bonds,
6 angles and 6 dihedrals are defined. Distance between all 1-4 pairs
is the same and all atoms have the same pair coeff.

I was expecting the LJ energy to be 3 x ( lj interaction energy of
single 1-4 pair ) = 3 x -0.5206 = -1.5617
( this is in agreement with another MD package which implements charmm ff )
but E_vdwl from 'thermo' output is off by a factor of 4 ( i.e. E_vdwl
= 12 x -0.5206 )

Any idea on how to fix this discrepancy ?

Thanks
Sandeep

ps:
I am using Jan-09 release of lammps. lj energy of -0.5206 for 1-4 pair
is for conformation in attached data file.

in.charmmLJ (976 Bytes)

data.charmmLJ (1.92 KB)

2

Sandeep,

The reason for the discrepancy is that you have 4 dihedrals defined for each of the three 1-4 pairs. So LAMMPS is computing each 1-4 pair interaction 4 times. One easy way to get agreement is to change your weighting factors from 1.0 to 0.25.

Paul

3

Cool .. scaling epsilon will work for me.
Knowing that the factor comes from the number of dihedral terms makes
it slightly less ad-hoc !

Thanks
Sandeep