Abstract
Reliable information on transition matrix elements of various property operators between molecular electronic states is of crucial importance for predicting spectroscopic, electric, magnetic and radiative properties of molecules. The finite-field technique is a simple and rather accurate tool for evaluating transition matrix elements of first-order properties in the frames of the Fock space relativistic coupled cluster approach. We formulate and discuss the extension of this technique to the case of transitions between the electronic states associated with different sectors of the Fock space. Pilot applications to the evaluation of transition dipole moments between the closed-shell-like states (vacuum sector) and those dominated by single excitations of the Fermi vacuum (the 1h1p sector) in heavy atoms (Xe and Hg) and simple molecules of heavy element compounds (I2 and TlF) are reported.
Highlights
In recent years, there has been a marked increase of interest in highly accurate theoretical data on excited electronic states and electronic transitions in molecules of heavy element compounds.Such data are of key importance for ultra-low temperature physics [1,2,3,4], searches for violation of time-reversal and spatial-parity symmetries of fundamental interactions in low-energy spectroscopic experiments [5] and high-resolution spectroscopy of short-lived radioactive atoms and molecules [6].The preparation of experimental studies and interpretation of their results require detailed knowledge of energetic, electric, magnetic, radiative and other molecular properties
The Fock space version of the relativistic coupled cluster method (FS RCC) is one of the most powerful tools of electronic structure modelling for heavy element compounds, widely used for obtaining accurate and reliable information on excited state potential energy surfaces and transition energies in molecules [7,8,9,10]
The expectation values of numerous important properties can be straightforwardly determined via using the finite-field technique, i.e., by numerical differentiation of the calculated energies with respect to the amplitude of appropriate perturbation. The extension of this approach to evaluating the transition matrix elements of first-order property operators proposed in [28,29] is based on the analysis of variations of the effective Hamiltonian eigenvectors induced by appropriate external field
Summary
There has been a marked increase of interest in highly accurate theoretical data on excited electronic states and electronic transitions in molecules of heavy element compounds. The expectation values of numerous important properties can be straightforwardly determined via using the finite-field technique, i.e., by numerical differentiation of the calculated energies with respect to the amplitude of appropriate perturbation (see [23,24,25,26,27] for recent applications) The extension of this approach to evaluating the transition matrix elements of first-order property operators proposed in [28,29] is based on the analysis of variations of the effective Hamiltonian eigenvectors induced by appropriate external field. Pilot applications to transition dipole moment calculations in heavy atoms and diatomic molecules of heavy element compounds are reported
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