  We study the electronic structure of small lithium clusters Li_n (n=2-8)
using the electronic stress tensor. We find that the three eigenvalues of the
electronic stress tensor of the Li clusters are negative and degenerate, just
like the stress tensor of liquid. This leads us to propose that we may
characterize a metallic bond in terms of the electronic stress tensor. Our
proposal is that in addition to the negativity of the three eigenvalues of the
electronic stress tensor, their degeneracy characterizes some aspects of the
metallic nature of chemical bonding. To quantify the degree of degeneracy, we
use the differential eigenvalues of the electronic stress tensor. By comparing
the Li clusters and hydrocarbon molecules, we show that the sign of the largest
eigenvalue and the differential eigenvalues could be useful indices to evaluate
the metallicity or covalency of a chemical bond.
