conjugation in molecular mechanics

 On Oct 4, ostlund' at \`hyper.com (Neil S. Ostlund) wrote:
 > Subject: conjugation in mol mech
 >
 > The standard molecular mechanics methods (MM2, Amber, CHARMm, etc)
 > assign parameters on the basis of the "atom type" of the relevant
 > atoms involved (4 atoms,in the case of a torsion) without consideration
 > of the bond type.  This results in the same torsional constants for the
 > SINGLE BOND in biphenyl at for the AROMATIC RING BONDS of Benzene!!
 I can't comment about MM2 or Amber; however, the currently released version
 of CHARMm does not predict that biphenyl _in vacuo_ is planar.  The actual
 angle given by a short test minimization is near 37 degrees; a vapor phase
 study has indicated an angle of "about 45 degrees".
 It's possible that an outdated version of the forcefield did predict a planar
 structure, or that the atoms were improperly typed, but the default typing
 from QUANTA with the current forcefield does produce planar phenyl rings with
 a 37 degree 'twist' in the single bond.
      Regards,
                 Bob Funchess
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