# [lammps-users] how to describe a potential similar to morse potential

Hi Axel,

Thank you so much for the suggestion. Unfortunately, I'm not sure I have understood your suggestion.
Yes, the second term is used to shift bond energy by constant. In our potential ,this shift constant is always imposed on bond energy. So, for all interatomic distances, this constant is added into the morse potential. Maybe I did not describe this potential clearly in the previous letter. I attached the equation of this potential with this email. If I just use the Morse potential equation, the constant will not be included. Could you kindly let me know what you mean like"just use morse bond potential as is"? Thanks again!

Best,

Lan

SRT force field.doc (16 KB)

Hi Axel,

Thank you so much for the suggestion. Unfortunately, I'm not sure I

dear lan,

then you should _really_ have a deep look into a text book.

what is the relation between a potential and the force?
or
how do you compute the force from a potential?

Yes, the second term is used to shift bond energy by constant. In our
potential ,this shift constant is always imposed on bond energy. So,
for all interatomic distances, this constant is added into the morse
potential. Maybe I did not describe this potential clearly in the
previous letter. I attached the equation of this potential with this

can or want to read them (i don't).

email. If I just use the Morse potential equation, the constant will
not be included. Could you kindly let me know what you mean like"just
use morse bond potential as is"? Thanks again!

i mean what i wrote. just answer one of the two questions
and then you can figure out the rest by yourself.

i give you one more hint: in principle you can add the impact
of the -D term to the simulation _after_ it is completed
with the existing morse bond potential.

cheers,
axel.

Dear Lan,

What Axel wanted to say is that since, it is a constant term, it will not effect the dynamics of your system at all. Think of it as a constant in the equation of a line y=mx+c. It is the slope m which govern molecular dynamics. You can have any value for c. You can add 100 other constant terms to the morse potential for that matter. It is only change your total energy by a certain amount. But the point is do you really care about the energy shift? Now, to get a feel of what is that shifted amount, you would have to count number of interactions when is equation is evolved and multiply it with D.

Regards,
Vikas