Highly Accurate Expectation Values Using High-Order Relativistic Coupled Cluster Theory

This work presents the automatic generation of analytic first derivatives of the energy for general coupled-cluster models using the tenpi toolchain. We report the first implementation of expectation values for CCSDT and CCSDTQ methods within the DIRAC program package for relativistic molecular calculations. As pivotal calculations, we focus on the electric field gradient (EFG) evaluated at the lithium nucleus in LiX (X = H, F, Cl) compounds, enabling the extraction of the nuclear electric quadrupole moment Q(⁷Li), and at the aluminum nucleus in AlY (Y = H, F, Cl, Br) compounds, for the determination of Q(²⁷Al). These high-order methods are applied to compute corrections for triple and quadruple excitations for the EFG, a crucial quantity for determining nuclear quadrupole moments. We obtain Q(²⁷Al) = 0.146598 ± 0.000001 b, in excellent agreement with the recommended value, and Q(⁷Li) = -0.038624 ± 0.000292 b, which is smaller than the currently recommended value, and suggests the need for further investigation.