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Re: [lammps-users] Reg: Intercation energy of co2 dimer and its atomic interactions
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Re: [lammps-users] Reg: Intercation energy of co2 dimer and its atomic interactions

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Mon, 7 Aug 2017 06:56:05 -0400

On Mon, Aug 7, 2017 at 4:32 AM, Zeeshan Ahmed <zeeshan.ahmed@...5589...> wrote:
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).



Zeeshan Ahmed
PhD Student (Mechanical Engineering)

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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.