Example: TS search using partial Hessian: C2H6 internal rotation
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"Download TS_C2H6.run"

   #! /bin/sh

   # Frequently when searching for a transition state, one needs an accurate second
   # derivatives matrix, a Hessian. An exact Hessian may be obtained analytically
   # but this may be very expensive for large molecules. In such cases it may be
   # beneficial to calculate Hessian matrix elements only for atoms directly
   # involved in the reaction for which a transition state is sought for. The rest
   # of the Hessian can then be approximated using a cheaper method.

   # In this example, a saddle point of the ethane internal rotation around C-C
   # bond is found. In principle, only hydrogen atoms contribute to the normal mode
   # we are interested in. Therefore we calculate a partial Hessian matrix
   # including hydrogen atoms only. For this purpose, the SelectedRegionForHessian
   # key is used.
   # In the calculation symmetry will be used.

   AMS_JOBNAME=Partial_Hessian $AMSBIN/ams <<eor
   System
     Symmetrize
     atoms
        C       0.000000000000       0.000000000000       0.767685465031
        C       0.000000000000       0.000000000000      -0.767685465031
        H       0.964354016767       0.347635559279       1.177128271450 region=hess
        H      -0.181115782790      -1.008972856410       1.177128271450 region=hess
        H      -0.783238233981       0.661337297125       1.177128271450 region=hess
        H      -0.500471876676       0.894626767091      -1.177128271450 region=hess
        H      -0.524533568868      -0.880734742626      -1.177128271450 region=hess
        H       1.025005445540      -0.013892024465      -1.177128271450 region=hess
     end
   end

   Task SinglePoint

   Properties
     NormalModes Yes
     SelectedRegionForHessian hess
   End

   Engine ADF
     title Ethane transition state search using partial Hessian
     beckegrid
       quality good
     end
     basis
       core Large
       type DZ
       CreateOutput Yes
     end
   EndEngine
   eor

   # After the Hessian is calculated, the resulting TAPE21 file is used for a
   # subsequent transition state search:

   AMS_JOBNAME=TS $AMSBIN/ams <<eor
   LoadSystem
      File Partial_Hessian.results/ams.rkf
   End

   Task TransitionStateSearch

   GeometryOptimization
      InitialHessian
         Type FromFile
         File Partial_Hessian.results/adf.rkf
      End
   End

   EngineRestart Partial_Hessian.results/adf.rkf

   Engine ADF
     title Ethane transition state search using partial Hessian
     beckegrid
       quality good
     end
     basis
       core Large
       type DZ
     end
   EndEngine

   eor

   # Important note: care should be taken to specify correct mode in the
   # TransitionState keyword. Because a significant part of the Hessian will not be
   # calculated exactly, it is possible that it will have more than one negative
   # eigenvalue, in which case the one we are interested in may not be the first
   # one. In such a case, one needs to specify the correct mode number in the
   # TransitionState keyword.
