A perturbative triples correction scheme to relativistic quadratic unitary coupled cluster method: Theory,...

We present a non-iterative triples correction to the relativistic quadratic unitary coupled cluster singles and doubles (qUCCSD) method, denoted as qUCCSD[T]. The method builds upon the Hermitian structure of the similarity-transformed Hamiltonian in the unitary coupled cluster method and can be derived by using perturbational truncation to the unitary coupled cluster energy functional. Relativistic effects are incorporated using the exact two-component atomic mean-field Hamiltonian, and the computational cost is further reduced through the frozen natural spinor and the Cholesky decomposition approximations. Benchmark results demonstrate that qUCCSD[T] outperforms previously proposed triples corrections to the unitary coupled cluster method and yields excellent agreement with experimental data and Full CI results. In addition, the method shows high accuracy in computing the bond dissociation enthalpies, molecular geometries, vibrational frequencies, ionization potentials, and electron affinities of heavy-element-containing systems. The new qUCCSD[T] method is competitive to popular CCSD(T) even on a classical computer.