Hi Tim,
thanks a lot for your work on OB.
Regarding the UFF implementation coded in OB I have a couple of questions but I
will start with just one.
The energy expression you used for trigonal planar coordination goes:
Triangular -> 2K_ijk/9 ( 1 + 4cos(t) * (1 +cos(t) ))
And the one in Rappé's paper (after Towhee's correction):
Triangular (UFF paper) -> K_ijk/9 ( 1 - cos(3t))
Those two expressions are not equivalent. Even more they differ a lot. A simple
plot would tell you that. They both have a minimum at 120 deg. Nevertheless
Rappé's version contains a minimum at 0. I agree with OB implementation that
this is not at all true but I want to know the origin of the expression you
have used. One important feature that can be obtained in Rappé's formulation is
that after taking the second derivative of the energy respect to the angle,
evaluated at the minumum always gives K_ijk for any coordination. This is not
recovered from the expression in OB (that value for trigonal planar is
4K_ijk/3).
Can you clarify this a bit for me?
Thanks a lot,
Fidel
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