  The relationships among the pressure P, volume V, and temperature T of
solid-state materials are described by their equations of state (EOSs), which
are often derived from the consideration of the finite-strain energy or the
interatomic potential.1-3 These EOSs consist of typically three parameters to
determine from experimental P-V-T data by fitting analyses. In the empirical
approach to EOSs, one either refines such fitting parameters or improves the
mathematical functions3-5 to better simulate the experimental data. Despite
over seven decades of studies on EOSs, none has been found to be accurate for
all types of solids over the whole temperature and pressure ranges studied
experimentally.3,6,7 Here we show that the simple empirical EOS, P =
{\alpha}1(PV) + {\alpha}2(PV)2 + {\alpha}3(PV)3, in which the pressure P is
indirectly related to the volume V through a cubic polynomial of the energy
term PV with three fitting parameters {\alpha}1 - {\alpha}3, provides accurate
descriptions for the P-vs-V data of condensed matter in a wide region of
pressure studied experimentally even in the presence of phase transitions
