Re: Conflicting energies for different force fields

[David Maxwell <dave-0at0-terminus.chem.yale.edu>]

 Hello,
 I am replying to the question about widely different energies from different
 force fields.
 It is normal for different force fields to differ for the same calculation
 on the same molecule, because the component energies in different force
 fields are not always calculated in the same manner.  In addition, a component
 like the non-bonded term may have different parameters, although the form is
 the same.
 Example
 -------
    A calculation with Macromodel for cyclohexane in chair form
                   Force field    E (kcal/mol)
                   ---------------------------
                       MM2            6.55
                       MM3            8.04
                      AMBER           1.72
                      OPLS            0.91
 These are values after optimizing with the default settings.
 Does this mean that the force fields are wrong in Macromodel?  Absolutely not!
 It is only useful to compare the relative energies between force fields.
 Example
 -------
    The energy difference between chair and twist-boat forms for cyclohexane
                   Force field   Rel. E (kcal/mol)
                   ---------------------------
                       MM2            5.36
                       MM3            5.76
                      AMBER           5.82
                      OPLS            6.77
 -Dave
                      *******************************
                      *        David Maxwell        *
                      *     Dept. of Chemistry      *
                      *       Yale University       *
                      * dave-0at0-terminus.chem.yale.edu *
                      *******************************