Radial orbitals were obtained from MCDHF calculations performed using the
Grasp2018 program package [
20,
36]. The RCI calculations based on configuration state function generators (CSFGs) [
22] were performed with the
Graspg program package [
37]. The details of the calculations, which were performed separately for each parity, are described below.
4.1. Hyperfine Interaction Constants from Energy-Driven Calculations
Starting with the odd-parity states, we performed an extended optimal level (EOL) [
38] Dirac–Hartree–Fock calculation of the 31 lowest states of the
configurations. The common
core shell of the MR configurations was not written out for simplicity. This calculation was followed by MCDHF calculations based on CSF expansions obtained by allowing single and double (SD) substitutions of orbitals in the MR configurations with orbitals in increasing orbital sets with the restriction that there should be at most one substitution from the
core shell (MR-SD VV + CV). Specified by the orbitals with the highest principal quantum numbers of each symmetry, and employing the non-relativistic notation, the orbital sets are listed in
Table 1.
The calculations were performed with the LBL approach, meaning that the orbitals from previous orbital sets were kept frozen, and only the outermost orbital of each symmetry of a new orbital set was optimized [
20]. The restriction to
symmetries can be motivated by the fact that hyperfine structure constants are less sensitive to orbitals with higher
l, which are pushed away from the important region in the vicinity of the nucleus due to the centripetal force [
39,
40].
The calculations for the even-parity states were performed in a similar way. We started with an extended optimal level (EOL) Dirac–Hartree–Fock calculation of the 50 lowest states of the
configurations. This calculation was followed by MCDHF calculations based on CSF expansions obtained by allowing single and double (SD) substitutions of orbitals in the MR configurations with orbitals in increasing orbital sets, with the restriction that there should be at most one substitution from the
core shell (MR-SD VV + CV). Specified by the orbitals with the highest principal quantum numbers of each symmetry, the orbital sets are given in
Table 2.
As discussed below, oscillations in the hyperfine interaction constants are related to the varying spatial localization of the correlation orbitals. That conclusion is substantiated by the mean radii
of the correlation orbitals, which are collected in
Table 3. The first set of values reported in the
Table 3 corresponds to the radii of the spectroscopic orbitals from the initial calculations. The second set refers to the radii of the outermost correlation orbitals within each orbital set. The data collected in
Table 3 reveal that the localizations, as measured by the mean radii, of the important correlation orbitals of
s symmetry are irregular and fluctuate in a region with the left boundary between
and
and the right boundary between
and
.
The MCDHF calculations were followed by RCI calculations based on CSF expansion obtained by allowing SD substitutions of orbitals in an enlarged MR set consisting of additional near-degenerate configurations with orbitals in the correlations sets listed in
Table 1 and
Table 2. The enlarged MR sets for the odd- and even-parity states were
and
The RCI calculations included the Breit interaction and the leading QED corrections (vacuum polarization and self-energy). To speed up the calculations, the CSFs were arranged in groups, with the same spin-angular structure spanned by CSFGs, thus allowing the spin-angular coefficients for all pairs of CSFs in a group or between two groups to be inferred from one or two ’template’ pairs. Compared to the ordinary RCI calculations, for which spin-angular integration is performed between all pairs of CSFs, the use of CSFGs reduces the execution time for the largest calculations (based on orbital set 7) by a factor of 10.
The calculated magnetic dipole hyperfine interaction constants are shown in
Table 4 and
Table 5 as functions of the increasing orbital sets. They exhibit several different convergence patterns. The hyperfine interaction constants of some odd-parity states (e.g.,
,
and
) oscillate heavily as the orbital set is increased, and they are not converged even after seven layers of correlation orbitals. Other states, e.g.,
and
are very stable and remain virtually unchanged as the orbital set is increased. In the case of even-parity states, the oscillations are not as pronounced, although there is a large change when going from orbital set 1 to orbital set 2. Worth noting are the large correlation effects for the
states and the slow convergence with respect to the increasing orbital set.
In the same
Table 4 and
Table 5, the few experimental values available are also reported for comparison with the present calculations. For the odd parity, the experimental value of Hirsch et al. [
41], obtained using a hydrogen maser technique, has an extraordinary accuracy, i.e.,
MHz. The corresponding error bars are not explicitly given in
Table 4 for this specific level to save room in the table. For all other levels, the hyperfine constant values have been determined using saturation laser spectroscopy by Jennerich et al. [
16]. As demonstrated by Carette et al. [
17], the presence of crossover resonances is problematic and the original spectra of Jennerich et al. had to be revisited to reconcile theory with observation. As written in their work, while the strong hyperfine lines are relatively easy to identify, the weak components are usually not. Most of the weak hyperfine lines were reinterpreted as crossover signals, producing hyperfine constant values that completely differ from the original ones. The uncertainty in these saturation spectroscopy measurements in the near-infrared is much larger than the one mentioned above for the ground level [
41].
4.2. Polarization Orbitals Merged with the Orbitals from Energy-Driven Calculations
To improve the convergence of the hyperfine interaction constants, the orbital basis should, based on the discussion in
Section 3, include orbitals specially targeted for capturing the important spin- and orbital-polarization effects. In this work, following [
14], we keep the spectroscopic orbitals from the first calculation of the odd-parity states frozen and optimize two
s and two
d orbitals on the weighted average of the targeted states based on CSF expansions formed by allowing single substitutions from the closed
subshell of the reference configurations. These orbitals and the corresponding CSFs capture most of the spin- and orbital-polarization effects of the
subshell. In the same way, we optimize two
s and two
d orbitals on the weighted average of the targeted states based on CSF expansions formed by allowing single substitutions from the closed
subshell of the reference configurations. These orbitals and the corresponding CSFs capture most of the spin- and orbital-polarization effects of the
subshell. Being separately optimized, the two sets of spin- and orbital-polarization orbitals are not orthogonal. Similar calculations of spin- and orbital-polarization orbitals were performed for the even states. The mean orbital radii of the spin- and orbital-polarization orbitals are reported at the bottom of
Table 3.
Due to orthogonality restrictions, the separately optimized and optimally localized polarization orbitals cannot be directly added into the orbital sets from the previous energy-driven LBL calculations. To deal with the orthogonality issue, we use the
rwfnrelabel program of
Grasp2018 to relabel the orbitals in the
polarization set (i.e., to change their principal quantum numbers which, together with the orbital quantum numbers, serve as the orbital identifiers), so that they carry unique identifiers, and on the hierarchical orbital list they eventually appear after the orbitals from the energy-driven LBL calculation. In the same way, we use the
rwfnrelabel program to relabel the orbitals in the
polarization set, so that they appear after the relabeled orbitals from the
polarization set. The relabeled polarization sets were added to, and orthogonalized against, the orbital sets from the energy-driven LBL calculation, forming final orthonormal orbital sets
Then, RCI calculations were performed for the odd- and even-parity states, based on CSF expansions obtained by allowing SD substitutions from the orbitals in the enlarged MR set (Equations (
12) and (
13)) to the correlation orbital sets in
Table 1 and
Table 2 generated by the energy-driven LBL calculations. These expansions were augmented with CSFs obtained by allowing S substitutions from the closed
subshell of the configurations in the enlarged MR set to the correlation orbitals in the
polarization set, and with CSFs obtained by allowing S substitutions from the closed
subshell of the configurations in the enlarged MR set to the correlation orbitals in the
and
polarization sets. It should be noted that the computational overhead incurred by including polarization orbitals is completely negligible. This becomes evident by comparing the number of CSFs generated from the largest orbital set in the energy-driven LBL calculation with the number of CSFs generated in the same expansion, but augmented with the CSFs based on the polarization sets, and accounting for the spin and orbital polarization. This comparison is reported in
Table 6 for the odd and even parities.
The hyperfine interaction constants from the RCI calculations with CSF expansions generated in the energy-driven LBL approach, and augmented with the CSFs based on the polarization sets, are shown in
Table 7 and
Table 8. When the spin- and orbital-polarization effects, based on separately optimized and optimally localized orbitals, are included, the convergence with respect to the increasing orbital set from the energy-driven LBL calculations becomes remarkably smooth.
The improvements in convergence of the hyperfine interaction constants for the
odd states and the
even states are graphically illustrated in
Figure 1. For the calculations using polarization orbitals, only three or four layers (orbital sets up to 3 or 4) from the energy-driven calculations are needed to achieve converged hyperfine constants. The convergence pattern for the
even state, however, differs from the other ones, and here, five or even six orbital layers from the energy-driven calculations are needed for convergence. As we will see later, this state is particularly sensitive to higher-order correlation effects as well as to the transformation to natural orbitals.
4.3. Higher-Order Correlation Effects and Transformation to Natural Orbitals
For accurate hyperfine interaction constants, higher-order electron correlation effects should be accounted for. This can be achieved by augmenting the CSFs from the previous RCI calculations with CSFs obtained by triple (T) and quadruple (Q) substitutions to increasing orbital sets. However, the orbital set cannot include more than a few layers, in order to keep the number of added CSFs at a manageable level. Alternatively, and this is the way we will follow, the CSFs from the previous RCI calculations can be augmented by CSFs obtained by SD substitutions from a larger MR set. For the odd-parity states, the enlarged MR set consisted of the 52 most energetically important configurations, with an accumulated squared weight of 99.5%. Allowing SD substitutions from these configurations to the
orbital set resulted in a final expansion of 18,565,748 CSFs. For the even-parity states, the enlarged MR set consisted of the 61 most energetically important configurations, again with an accumulated squared weight of 99.5%. Allowing SD substitutions from these configurations to the
orbital set resulted in an expansion of 16,431,197 CSFs. The hyperfine interaction constants from these calculations are collected in
Table 7 and
Table 8, in the columns with the TQ header. The influence of the higher-order correlation effects is rather small, with the exception of a few states. For the
odd state, the hyperfine interaction constant is reduced by 7 %, bringing the value in better agreement with the experimental value. For the
even states, the hyperfine interaction constants are increased by 17% and 11%, respectively. Incidentally, for the latter states the total correlation effects were largest. Similar observations of their sensitivity to higher-order correlation effects were made by Jönsson et al. [
18], based on MCHF calculations.
In the LBL calculations, as discussed in
Section 1, orbitals are kept frozen from a fully variational EOL calculation of the targeted states, based on the most important reference configurations, and are not allowed to relax in response to the introduced layers of correlation orbitals. More specifically, the
valence orbitals do not respond to the effects which result from the interaction with the
core (core–valence). The transformation to natural orbitals accounts for these effects, and in
Table 9 we provide orbital radii after the transformation.
For the odd states, the main effects are the expansion of the
orbital and the contraction of the
orbital. The effects on the hyperfine constants, as seen from
Table 7, are small, the exception being the
state, for which the constant increases by 2.6%. The hyperfine interaction constant for this state results mainly from large and canceling contributions from the spin polarization of the
and
closed subshells, and the effect of the orbital transformation is to change this balance slightly. For the even states, the main effects are the contraction of the
orbital and the expansion of the
orbital. The effects of the contraction can be seen for the
states, for which the constants increase by 3.3% and 2.6%, respectively. The effects for the remaining states are small; see
Table 8. The final values of the hyperfine interaction constants are in good agreement with both the extensive MCHF calculations of Jönsson et al. [
18] and with the experimental values deduced by Carette et al. [
17] from the spectral profiles published by Jennerich et al. [
16], but revisiting their analysis on the basis of crossover signals. Using the presently available set of experimental data, some differences with respect to observation are, however, worth noting. The hyperfine interaction constants for the
even states are strongly affected by electron correlation effects. Even using specifically targeted spin- and orbital-polarization orbitals, the values change by factors of 3 and 2, respectively, as the energy-driven orbital set is increased. These states are also sensitive to higher-order correlation effects and to transformation to natural orbitals. Both the results of the current calculations and of the MCHF calculations in Ref. [
18] are 10% smaller than the respective experimental values.