  The development of a quadratic unitary coupled-cluster singles and doubles
(qUCCSD) based self-consistent polarization propagator method is reported. We
present a simple strategy for truncating the commutator expansion of the UCC
transformed Hamiltonian $\bar{H}$. The qUCCSD method for the electronic
ground-state includes up to double commutators for the amplitude equations and
up to cubic commutators for the energy expression. The qUCCSD excited-state
eigenvalue equations include up to double commutators for the singles-singles
block of $\bar{H}$, single commutators for the singles-doubles and
doubles-singles blocks, and the bare Hamiltonian for the doubles-doubles block.
Benchmark qUCCSD calculations of the ground-state properties and excitation
energies for representative molecules demonstrate significant improvement of
the accuracy and robustness over the previous UCC3 scheme derived using
M{\o}ller-Plesset perturbation theory.
