I have used ab initio MO to calculate the interaction energy of CO2 dimer of different configurations at various distances. My question is how to get C-C, C-O and C-O atomic interactions from the obtained energy graph as I need them to create input file for LAMMPS. Also, the interaction energies curves obtained for different CO2 dimer configurations are different (i.e for two parallel co2 molecules, for T shapes of two co2 molecules etc). So how to extract LJ parameters i.e., sigma and epsilon from these obtained interaction energies of CO2 dimer. Please help

I have used ab initio MO to calculate the interaction energy of CO2 dimer

of different configurations at various distances. My question is how to get

C-C, C-O and C-O atomic interactions from the obtained energy graph as I

need them to create input file for LAMMPS. Also, the interaction energies

curves obtained for different CO2 dimer configurations are different (i.e

for two parallel co2 molecules, for T shapes of two co2 molecules etc). So

how to extract LJ parameters i.e., sigma and epsilon from these obtained

interaction energies of CO2 dimer. Please help

the kind of help you need is *far* beyond the scope of this mailing list.

first off, at what level of theory did you do your ab initio calculations

and with what kind of basis set? i've been doing such calculations myself

some time back and it took a *lot* of care, and compute power to get

converged results. CO2 is very tricky.

second, that parallel and t-shape orientation result in different energies

is no surprise. while CO2 molecules have no charge or dipole moment, they

do have a non-negligible quadrupole moment.

third, force field parameters are obtained by various fitting procedures.

there are *lot* of publications on how different force fields or parameter

sets are derived. there is no single general procedure. each force field

has different approaches, and for specific sets of atoms/molecules, there

are often special force fields with better representation of the properties

than for generalized force fields.

fourth, it will be difficult to get a good model without also fitting

partial charges.

fifth, why on earth do you attempt doing a force field parameterization?

something that you obviously have no experience in and for a compound for

which there are plenty of suitable parameterizations available in the

published literature. you are not the first person attempting to do

simulations with CO2, not by a long shot. so why don't you just do the

smart thing and do a thorough survey of the published literature, identify

the most popular parameterizations of CO2 used for your simulation

conditions, test a few of those and pick the one that fits your needs best.

attempting your own parameterization in a fashion that you can depend on

it, requires that you have to spend much more time, as you have to learn

how to do force field parameterization (well) first and also spend a lot of

time characterizing your parameterization and that it represents the

properties you want to be represented sufficiently well (and compare to

competing parameter sets, so you do have to do the literature survey

anyway).

axel.