# AMBER Archive (2004)Subject: RE: AMBER: force field

From: Ross Walker (ross_at_rosswalker.co.uk)
Date: Fri Jan 02 2004 - 06:55:24 CST

Dear Hungie

This is simply a matter of symantics - The force is the differential of the
potential energy and thus if the potential energy is defined as:

E = K(r-req)^2

Then the force is:

F = 2K(r-req)

Often people refer to K as being the force constant which in the above
example isn't strictly correct. The force constant is 2K. If however you
express the energy in terms of K/2 then the force is K(r-req) and so K in
this case is the force constant.

So really it is just how you choose to define the force constant. Since K is
a constant the only difference is in what you define it's value to be. I.e
in the first example the value of K would be half that of the second
example. As long as you are consistent in your definition and assignment of
the constants it doesn't strictly matter which form you use.

I hope this helps.
All the best
Ross

/\
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|\oss Walker

| Department of Molecular Biology TPC15 |
| The Scripps Research Institute |
| Tel:- +1 858 784 8889 | EMail:- ross_at_rosswalker.co.uk |
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-----Original Message-----
From: owner-amber_at_scripps.edu [mailto:owner-amber_at_scripps.edu] On Behalf Of
A. Hungie
Sent: 02 January 2004 04:29
To: amber_at_scripps.edu
Subject: AMBER: force field

Dear All,

As I have seen in the literatures, the feature of force field of AMBER, for
example bond term is

summation { Kb(l-l0)^2}

whereas in some book it is

summation {Kb/2(l-l0)^2} , i.e. there is a term of 1/2

Could you please tell me why they are different?

Best Regards,
Hungie

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