Subject: Universal Force Field (UFF)

From: "derosa" <[email-masked]>
Date: Tue, 16 Oct 2001 21:42:07 -0400

Dear all:

        I am trying to figure out how the Universal Force Field is
parameterized in Cerius2, particularly the torsion and the inversion
interactions.  My source is the Paper of Rappe et al J. Am. Chem. Soc. 1992
114, 10024 and the set of parameter shown in the force field UFF 1.02.

        According to what I understood from that paper, the torsional energy
is given by

E=1/2 V[1-cos(n*phi0)*cos(n*phi)]

It is a little obscure for me how this parameters are defined
For sp2 centers connected to another sp2 center n=2 and phi0 is 180, which
is my case.  so

E=1/2 V [1-cos(2*phi)]

My first question is how V is defined.  In the mentioned paper there are two
different sum rules depending on whether the center is sp2 or sp3 however
only for sp3 centers V is defined as a function of a "torsional barrier
factor" of each atoms which is the only parameter (I could find) in the
force field.  SO how is V defined (in cerius2) for sp2 centers, which is the
sum rule I should use?.

With respect to the inversion, Rappe et al indicates that the inversion
energy is given by

E=Kijkl( Co + C1 sin(Gamma)+ C2 cos(2Gamma))

However UFF, as implemented in cerius2 only offers two parameters, the
inversion barrier and the equilibrium inversion angle for each atoms.  How
the other parameters are obtained?, is the previous formula the one that is
used in cerius2?.

Any clarifying comment will be much appreciated.  You can send the answers
directly to me at [email-masked] I will summarize if there is any
interest.

Thank you very much for your help,

Pedro