A Reduced Cost Four-Component Relativistic Unitary Coupled Cluster Method for Molecules

We present a four-component relativistic unitary coupled cluster method formolecules. We have used commutator-based non-perturbative approximation usingthe ''Bernoulli expansion'' to derive an approximation to the relativisticunitary coupled cluster method. The performance of the full quadratic unitarycoupled-cluster singles and doubles method $\left ( qUCCSD \right )$, as wellas a perturbative approximation variant $\left ( UCC3 \right )$, has beenreported for both energies and properties. It can be seen that both methodsgive results comparable to those of the standard relativistic coupled clustermethod. The qUCCSD method shows better agreement with experimental results dueto better inclusion of the relaxation effects. A natural spinor-based scheme toreduce the computation cost of relativistic UCC3 and qUCCSD methods has beendiscussed.