I have built a system with fix boundary conditions containing two CO2 molecules at different geometries to calculate the total energy of the system. I want to check whether the parameters from LB mixing rule gives the correct energy or not by comparing the total energy values obtain from DFT. When I varied the distance from 2 Angstrom to 6 Angstrom without doing any optimization to calculate the energies , the values varies 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 equals to zero to simply calculate the total energy of the two molecule system. I was expecting the energy values to fluctuate around 1496 to 1480 kcal/mol, so that I can do paramterization of sigma and epsilon obtained from LB mixing rule. Please help!