  We present the theory, implementation, and benchmark results for a frozen
natural spinors-based lower scaling four-component relativistic coupled cluster
method. The natural spinors are obtained by diagonalizing the one-body reduced
density matrix from a relativistic MP2 calculation based on four-component
Dirac-Coulomb Hamiltonian. The correlation energy in the coupled cluster method
converges more rapidly with respect to the size of the virtual space in the
frozen natural spinor basis than that observed in the standard canonical
spinors obtained from the Dirac-Hartree-Fock calculation. The convergence of
properties is not smooth in the frozen natural spinor basis. However, the
inclusion of the perturbative correction smoothens the convergence of the
properties with respect to the size of the virtual space in the frozen natural
spinor basis and greatly reduces the truncation errors for both energy and
properties calculations. The accuracy of the frozen natural spinor based
coupled cluster methods can be controlled by a single threshold and is a black
box to use.
