Universal Force Field (UFF)

["derosa" <derosa "-at-" engr.sc.edu>]

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 derosa "-at-" engr.sc.edu I will summarize if there is any
 interest.
 Thank you very much for your help,
 Pedro