Ján Šimunek | Search arXiv

arXiv2026

Precise Determination of Electric Dipole Polarizabilities of Sr Clock States Using Linear-Response and Equation-of-Motion Relativistic Coupled-Cluster Methods

We present accurate values of the electric dipole polarizabilities ($α$) of the ground and excited states of Sr using four-component relativistic coupled-cluster (RCC) theory within the linear-response (LR) and equation-of-motion (EOM) frameworks. We first investigated accuracy of the ground-state polarizability using the LR-RCC method in both spherical and Cartesian coordinate representations. Upon obtaining consistent results for the ground state, we determine the polarizabilities of the excited states using the EOM-RCC method within the finite-field approach. The uncertainties in our results are assessed by examining the completeness of the basis set, higher-order relativistic corrections, and contributions from higher-level excitation configurations beyond the truncation in our RCC method. We also discuss various procedures for extrapolating contributions to the complete basis set limit and examine their respective limitations. Our calculated ground-state polarizability is found to be inconsistent with the latest atomic calculation, but agrees with other calculations and experimental value. Using the differential $α$ value from the EOM-RCC method, we estimate the fractional black-body-radiation-induced frequency shift of the Sr $^{1}$S$_{0}$$\rightarrow$$^{3}$P$_{0}$ clock transition to be $-5.40\times10^{-15}$.

physics.chem-ph↗

arXiv2025

A perturbative triples correction to relativistic Quadratic Unitary Coupled Cluster Method: Theory, Implementation and Benchmarking

We present a perturbative 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 unitary ansatz and employs a many-body perturbation theory framework to consistently include the effects of triple excitations without the need for computationally intensive iterative procedures. Relativistic effects are incorporated using the exact two-component atomic mean-field (X2CAMF) Hamiltonian, and the computational cost is further reduced through the frozen natural spinor (FNS) and Cholesky decomposition (CD) approximations. Benchmark results demonstrate that qUCCSD[T] outperforms previously proposed triples corrections to the unitary coupled cluster method in the clasical computing regime and yields excellent agreement with experimental data and Full CI benchmarks. Specifically, the method shows high accuracy in computing bond dissociation enthalpies, molecular geometries, vibrational frequencies, ionization potentials, and electron affinities of heavy-element-containing systems.

physics.chem-ph↗