  Understanding thermodynamics in liquids at the atomic level is challenging
because of strong atomic interactions and lack of symmetry. Recent prior
theoretical works have focused on describing heat capacity of liquids in terms
of phonon-like excitations but often rely on fitting parameters and ad hoc
assumptions. In this work, we perform microscopic analysis on instantaneous
normal modes and velocity autocorrelations on molecular dynamics simulations of
single element systems over wide ranges of temperature (up to $10^8$ K) and
pressure (up to 1 TPa). Our results demonstrate that heat capacity of liquids
can be described by a combination of both solid-like and gas-like degrees of
freedom, leading to a unified framework to describe heat capacity of all three
phases of matter: solid, liquid, and gas.
