Subject: TS BY GEOM OPT, KCAL AND HARTREES--RESPONSE

From: "E. Lewars" <[email-masked]>
Date: Thu, 30 Jul 1998 11:54:30 -0400 (EDT)

Thurs July 30

Yury Voloshin asked
1) How can a geom opt (rather than a TS opt) give a transition state?

2) How can one convert semiempirical energies, in kcal/mol, to hartrees, for
   comparison with ab initio values?

-----------

1)  If your input geom has the same symmetry as a TS (of about the about the
    same geom), a geom opt will take it to that TS. Examples:

          H
          |
          |           geom opt
   F ---- C ---- F      ----->   TS for (degenerate) Sn2 displacement
         -  \
        -     \
      H        H

         D3h

                                geom opt
    D3h ethane (eclipsed)  ----->    TS for interconversion of
                                     D3d (staggered) conformations

        N
    N       N
                  AM1 geom opt
    N       N     ---------------->   TS for degenerate interconv. of
        N                             3 N2

       D6h

     One can often find a TS this way. Of course a TS at one computational level
  may be a minimum or a higher-order saddle point at a fifferent level.
-----

 2)  As you imply, semiemp and ab initio E's refer to different things and so
     can't be directly compared. However, chemistry studies energy
     _differences_, not energies. To covert a semiemp deltaE to hartrees,
     divide by 627.5:

       H      H              H      CH3                    Z    -------
        \    /                \    /                             /|\
         C==C                  C==C                               |
        /    \                /    \      (AM1)                   |
      CH3     CH3           CH3     H                    deltaE   |
                                                                  |
        -2.28 kcal/mol         -3.35 kcal/mol               E  --------
            Z                        E

                                deltaE = (-2.28 - (-3.35))/627.5 hartrees

                                       =1.07/627.5  h = 0.00171 h

       Also: deltaE h/molecule x 2626 = deltaE kJ/mol
============

      E. Lewars
============================