# [gmx-users] dimension

Nagy, Peter I. PNAGY at UTNet.UToledo.Edu
Mon Jan 19 17:14:02 CET 2004

```Dear Gromacs Users,

From the manual eq 4.46, I understand that the dimension of the harmonic bending force constant, V = (1/2)*k(theta,harm)*[(delta theta)}**2] is kJ/mol/(deg**2), thus the energy change is 1/2*k by changing the bond angle by 1 deg.

What is the meaning of the cosine based bending constant (eq. 4.48)? The manual also says that

k(theta)[sin(theta0)**2] = k(theta, harm)

Since sin(theta) (and also cos(theta) is a dimensionless number,
the dimension of k(theta) should be equal to the dimension of k(theta, harm),
which is kJ/[mol*(deg**2)]. But from 4.48 the physical meaning of k(theta)
would be the double of the potential energy change by changing the bond angle characterized by [cos(theta) -cos(theta0)] = +1 or -1

Thanks for the help.

Peter Nagy
The University of Toledo
pnagy at utnet.utoledo.edu
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