# A frequency calculation is performed using the mobile block Hessian
# (MBH)method. The coordinates in the ATOMS section should be the partially
# optimized coordinates (or the fully optimized coordinates would work too).
# Example input how to do a block constraint:
# C 0.000000 0.000000 0.000000 b=b1
# H 0.634671 0.634671 0.634671 b=b1
# H -0.634671 -0.634671 0.634671 b=b1
# H -0.634671 0.634671 -0.634671 b=b1
# H 0.634671 -0.634671 -0.634671
# block b1
# Such geometry optimization will not be discussed here any further. The next
# input for ADF shows how to perform a frequency calculation with MBH.
C 0.000000 0.000000 0.000000 b=b1
H 0.634671 0.634671 0.634671 b=b1
H -0.634671 -0.634671 0.634671 b=b1
H -0.634671 0.634671 -0.634671 b=b1
H 0.634671 -0.634671 -0.634671 b=b2
# The flag b=b1 in the ATOMS section adds the label 'b1' to some of the atoms.
# The four atoms labeled 'b1' will be considered as a block with fixed internal
# In the GEOMETRY section, a Mobile Block Hessian calculation is requested by
# using the FREQUENCIES and MBH keywords. Here the atoms with label 'b1' are
# selected to be in the same mobile block. The position/orientation of the block
# are supposed to be optimized in a preceding partial optimization run. In the
# vibrational analys, the block 'b1' is only allowed to vibrate as a whole. The
# number of resulting modes/frequencies is 3 for the fifth atom plus 6 for the
# block 'b1' (3 position/3 orientation), resulting in 9 frequencies in total.
# Since 6 of those frequencies are zero due to translational and rotational
# invariance of the system, one will find 3 non-zero characteristic frequencies
# in the output. In practice with ADF not exactly 6 zero's are found, but they
# are close to zero.
# The quality of the frequencies/modes depends largely on the block choice. Best
# results are obtained when grouping atoms in a block if those atoms are known
# to form rather rigid structures. For instance, grouping the 11 atoms of
# benzene side group into a block, will usually result in representative
# frequencies. In this example the block choice is only illustrative for the