  In addition to Goldstone phonons that generically emerge in the low-frequency
vibrational spectrum of any solid, crystalline or glassy, structural glasses
also feature other low-frequency vibrational modes. The nature and statistical
properties of these modes -- often termed `excess modes' -- have been the
subject of decades-long investigation. Studying them, even using
well-controlled computer glasses, has proven challenging due to strong spatial
hybridization effects between phononic and nonphononic excitations, which
hinder quantitative analyses of the nonphononic contribution ${\cal D}_{\rm
G}(\omega)$ to the total spectrum ${\cal D}(\omega)$, per frequency $\omega$.
Here, using recent advances indicating that ${\cal D}_{\rm G}(\omega)\!=\!{\cal
D}(\omega)-{\cal D}_{\rm D}(\omega)$, where ${\cal D}_{\rm D}(\omega)$ is
Debye's spectrum of phonons, we present a simple and straightforward scheme to
enumerate nonphononic modes in computer glasses. Our analysis establishes that
nonphononic modes in computer glasses indeed make an additive contribution to
the total spectrum, including in the presence of strong hybridizations.
Moreover, it cleanly reveals the universal ${\cal D}_{\rm
G}(\omega)\!\sim\!\omega^4$ tail of the nonphononic spectrum, and opens the way
for related analyses of experimental spectra of glasses.
