  We present the theory and the implementation of a low-cost four-component
relativistic equation of motion coupled cluster method for ionized states based
on frozen natural spinors. A single threshold (natural spinor occupancy) can
control the accuracy of the calculated ionization potential values. Frozen
natural spinors can significantly reduce the computational cost for valence and
core-ionization energies with systematically controllable accuracy. The
convergence of the ionization potential values with respect to the natural
spinor occupancy threshold becomes slower with the increase in basis set
dimension. However, the use of a natural spinor threshold of 10-5 and 10-6
gives excellent agreement with experimental results for valence and core
ionization energies, respectively.
