  The values of molecular polarizabilities and softnesses of the twenty amino
acids were computed ab initio (MP2). By using the iterative Hirshfeld scheme to
partition the molecular electronic properties, we demonstrate that the values
of the softness of the side chain of the twenty amino acid are clustered in
groups reflecting their biochemical classification, namely: aliphatic, basic,
acidic, sulfur containing, and aromatic amino acids . The present findings are
in agreement with previous results using different approximations and
partitioning schemes [P. Senet and F. Aparicio, J. Chem. Phys. 126,145105
(2007)]. In addition, we show that the polarizability of the side chain of an
amino acid depends mainly on its number of electrons (reflecting its size) and
consequently cannot be used to cluster the amino acids in different biochemical
groups, in contrast to the local softness. Our results also demonstrate that
the global softness is not simply proportional to the global polarizability in
disagreement with the intuition that "a softer moiety is also more
polarizable". Amino acids with the same softness may have a polarizability
differing by a factor as large as 1.7. This discrepancy can be understood from
first principles as we show that the molecular polarizability depends on a
"softness dipole vector" and not simply on the global softness.
