Reg: Interaction energy calculation between two molecules in a system

Dear all,

I have built a system with fix boundary conditions containing two rigid gas molecules at different geometries to calculate the total energy of the system and also the interaction energy between the molecules. I didn’t do any minimization and didn’t consider bond energy angular energy etc. as I am just interested in calculating the energy of rigid molecules at various distances. When I varied the distance from 2 Angstrom to 6 Angstrom without doing any optimization to calculate the energies, the values vary from 1496 kcal/mol at 2.5 Angstrom to -1.9 kcal/mol at 6 Angstrom. I have used fix rigid/nve keeping force and torque flags equal to zero to simply calculate the energy of the two molecule system. Please see the code attached. Did I do any thing wrong in the code?

units real
atom_style full

setup problem

dimension 3
boundary f f f
read_data data.gas_dimer
pair_style lj/cut 12
pair_coeff 1 1 1.5890 2.785
pair_coeff 1 2 2.6892 2.8995
pair_coeff 2 2 4.5512 3.014

#group A molecule <> 1 3
#group B molecule <> 4 6

#fix 1 all nve
#fix 2 A setforce 0.0 0.0 0.0
#fix 4 B setforce 0.0 0.0 0.0

fix NVE all rigid/nve molecule force 12 off off off torque 12 off off off

thermo_style custom step etotal epair temp vol press density

timestep 1

run 10000

It is perfectly reasonable – why do you think there is something wrong?