#! /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 AnalyticalFreq block key
# is used. In this block, a NUC keyword is added specifying that the second
# derivatives are calculated for atom 3 (and its symmetry-equivalents) only.
# Note that the Hessian matrix elements between symmetry-equivalent atoms, for
# example between 3,H and 4.H are also calculated. The rest of the matrix is
# estimated using the default method.
"$ADFBIN/adf" <