Geometry optimizations with spinor-based relativistic coupled-cluster theory.

Development of analytic gradients for relativistic coupled-cluster singles and doubles augmented with a non-iterative triples [CCSD(T)] method using an all-electron exact two-component Hamiltonian with atomic mean-field spin-orbit integrals (X2CAMF) is reported. This enables efficient CC geometry optimizations with spin-orbit coupling included in orbitals. The applicability of the implementation is demonstrated using benchmark X2CAMF-CCSD(T) calculations of equilibrium structures and harmonic vibrational frequencies for methyl halides, CHX (X = Br, I, and At), as well as calculations of rotational constants and infrared spectrum for RaSH, a radioactive molecular ion of interest to spectroscopic study.