# What is the correct K-theta value of the EPM2 (flexible version) Model for CO2 in LAMMPS?

Hello everyone!

I am going to use the flexible version of the EPM2 model for CO2 and I have simple question but I just need to make sure that everything is correct.

As in the paper by Harris & Yung (1995), the value for the K-theta is 1236 kj/mol/rad2 (295.411 Kcal/mol/rad2) and the formula is E=1/2K(theta-theta0)^2. However, in the LAMMPS document the formula which is used for calculating the harmonic potential of the angles is defined as: E=K(theta-theta0)^2, and it is mentioned that the usual 1/2 factor is included in K.

So, what value should be specified in lammps data/input file, 295.411 or 147.7?

Thanks

Please note that this is a discussion forum and not a â€śuser input validation serviceâ„˘â€ť.

If someone would tell you one or the other value (or give you a third option) how much more do you know whether this is correct or not? What if the person responding is someone that likes pranks or is a troll? What if you use the wrong recommendation and all your simulations are tainted and your work wasted?

What you have to do is to apply the scientific method: if you are uncertain about something, set up a simple test, make a hypothesis from the theory, see if that hypothesis is validated or not.
In this case, you can just set up a system with three atoms and the necessary bonds/angles, use pair and bond style zero, and angle style harmonic. Now you can use different geometries and compute/dump the forces (using the â€śrun 0â€ť and â€śwrite_dumpâ€ť commands) on the atoms and compare them to forces you have computed with pencil and paper from the potential energy function. This is good practice and something that everybody should do when working with unknown software and new parameters.

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