# Example: TS search using partial Hessian: C2H6 internal rotation¶

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
File Partial_Hessian.results/ams.rkf
End

GeometryOptimization
InitialHessian
Type FromFile
End
End

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.