  We present a four-component relativistic unitary coupled cluster method for
molecules. We have used commutator-based non-perturbative approximation using
the ''Bernoulli expansion'' to derive an approximation to the relativistic
unitary coupled cluster method. The performance of the full quadratic unitary
coupled-cluster singles and doubles method \left ( qUCCSD \right ), as well as
a perturbative approximation variant \left ( UCC3 \right ), has been reported
for both energies and properties. It can be seen that both methods give results
comparable to those of the standard relativistic coupled cluster method. The
qUCCSD method shows better agreement with experimental results due to better
inclusion of the relaxation effects. A natural spinor-based scheme to reduce
the computation cost of relativistic UCC3 and qUCCSD methods has been
discussed.
