Relativistic coupled cluster calculation of Mössbauer isomer shifts of iodine compounds

Abstract

Mössbauer isomer shifts of 129 I and 127 I in the ICl, IBr and I 2 molecules are studied. Filatov's formulation is used, based on calculating the electronic energy change of the two systems involved in the Mössbauer γ transition, the source and absorber. The energy difference between the transitions in the two systems determines the shift. The effects of relativity and electron correlation on the shifts are investigated. The exact two-component (X2C) and the four-component relativistic schemes give virtually identical results; the non-relativistic approach yields about 50% of the relativistic shifts. Electron correlation is included by coupled-cluster singles-and-doubles with perturbative triples [CCSD(T)]; it reduces Hartree-Fock shifts by 15%-20%. Basis sets are increased until the isomer shifts converge. The final results, calculated with the converged basis in the framework of the X2C Hamiltonian and CCSD(T) correlation, give an agreement of 10% or better with experimental data.

The DIRAC code for relativistic molecular calculations

“…86 The MRCI and CC methods implemented in DIRAC that account for more dynamic correlation have, combined with the experiment, provided reference values for properties such as nuclear quadrupole moments, 223,224 hyperfine structure constants, 225 and Mössbauer isomer shifts. 226 DIRAC also provides theoretical input for spectroscopic tests of fundamental physics, within both the standard model of elementary particles 227 and tests of Beyond Standard Model (BSM) theories that give rise to electric dipole moments of fermions. 141,143,228 In recent years, DIRAC has been extended to include several models for large environments: PCM, PE, and FDE, which opens new perspectives.…”

The DIRAC code for relativistic molecular calculations

“…81 Methods implemented in DIRAC that account for more dynamic correlation have, combined with experiment, provided reference values for properties such as nuclear quadrupole moments, 214,215 hyperfine structure constants 216 and Mössbauer isomer shifts. 217 DIRAC also provides theoretical input for spectroscopic tests of fundamental physics, both within the Standard Model of elementary particles 218 as well as tests of Beyond Standard Model (BSM) theories which give rise to electric dipole moments of fermions. 135,137,219 In recent years, DIRAC has been extended to include several models for large environments: PCM, PE and FDE, which opens new perspectives.…”

Implementation of Relativistic Coupled Cluster Theory for Massively Parallel GPU-Accelerated Computing Architectures

“…The standard approaches to compute ground-state energies, molecular properties, and electronically excited states have all been generalized to relativistic theory as well, yielding methods that can provide very high accuracy in the electronic structure part of a calculation. This is demonstrated in numerous small-molecule applications − for which steep scaling with the system size of the coupled cluster algorithm is not an issue. This rapid increase in computational requirements does, however, in practice, prevent the application of relativistic CC with a fully spin–orbit-coupled reference wave function to systems that contain more than about ten atoms.…”