  Computer simulations show that liquids of molecules with harmonic
intramolecular bonds may have "pseudoisomorphic" lines of approximately
invariant dynamics in the thermodynamic phase diagram. We demonstrate that
these lines can be identified by requiring scale invariance of the
inherent-structure reduced-unit low-frequency vibrational spectrum evaluated
for a single equilibrium configuration. This rationalizes why excess-entropy
scaling, density scaling, and isochronal superposition apply for many liquids
with internal degrees of freedom.
