  Recently, it was shown that a simple empirical formula, in terms of the mass
and valence nucleon numbers, can describe the main trends of the yrast
excitation energies of the natural parity states up to $10^+$ in even-even
nuclei throughout the entire periodic table. The same empirical formula was
applied to the yrast excitation energies of unnatural parity states including
$1^+$, $2^-$, $3^+$, $4^-$, $5^+$, $6^-$, $7^+$, $8^-$, $9^+$, $10^-$, and
$11^+$ in even-even nuclei. Although the overall character of the effective
residual interaction for the unnatural parity states was quite different to
that of the natural parity states, the same form of the empirical formula was
found to hold reasonably well for the yrast excitation energies of the
unnatural parity states.
