DIRAC is a freely distributed general-purpose program system for one-, two-, and four-component relativistic molecular calculations at the level of Hartree–Fock, Kohn–Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, electron propagator, and various flavors of coupled cluster theory. At the self-consistent-field level, a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows for the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding model, and the frozen density embedding model.

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The DIRAC code for relativistic molecular calculations

Laser cooling typically doesn't work on asymmetric molecules, but a new analysis shows that it is possible for certain species, paving the way for a new era for molecular probes.

https://link.aps.org/pdf/10.1103/PhysRevX.10.031022

Molecular Asymmetry and Optical Cycling: Laser Cooling Asymmetric Top Molecules

Precision measurements of Schiff moments in heavy, deformed nuclei are sensitive probes of beyond standard model T, P violation in the hadronic sector. While the most stringent limits on Schiff moments to date are set with diamagnetic atoms, polar polyatomic molecules can offer higher sensitivities with unique experimental advantages. In particular, symmetric top molecular ions possess K doublets of opposite parity with especially small splittings, leading to full polarization at low fields, internal comagnetometer states useful for rejection of systematic effects, and the ability to perform sensitive searches for T, P violation using a small number of trapped ions containing heavy exotic nuclei. We consider the symmetric top cation ^{225}RaOCH_{3}^{+} as a prototypical and candidate platform for performing sensitive nuclear Schiff measurements and characterize in detail its internal structure using relativistic ab initio methods. The combination of enhancements from a deformed nucleus, large polarizability, and unique molecular structure make this molecule a promising platform to search for fundamental symmetry violation even with a single trapped ion.

/pdf/probing-fundamental-symmetries-of-deformed-nuclei-in-1q0xudjt1z.pdf

Probing Fundamental Symmetries of Deformed Nuclei in Symmetric Top Molecules.

Precision measurements in molecules have advanced rapidly in recent years through developments in techniques to cool, trap, and control. The complexity of molecules makes them a challenge to study, but also offers opportunities for enhanced sensitivity to many interesting effects. Polyatomic molecules offer additional complexity compared to diatomic molecules, yet are still "simple" enough to be laser-cooled and controlled. While laser cooling molecules is still a research frontier itself, there are many proposed and ongoing experiments seeking to combine the advanced control enabled by ultracold temperatures with the intrinsic sensitivity of molecules. In this perspective, we discuss some applications where laser-cooled polyatomic molecules may offer advantages for precision measurements of fundamental physics, both within and beyond the Standard Model.

/pdf/polyatomic-molecules-as-quantum-sensors-for-fundamental-3odtatx62j.pdf

Polyatomic Molecules as Quantum Sensors for Fundamental Physics

We present the first experimental demonstration of radiation pressure force deflection and direct laser cooling for barium monohydride (BaH) molecules resulting from multiple photon scattering. Despite a small recoil velocity (2.7 mm/s) and a long excited state lifetime (137 ns), we use 1060 nm laser light exciting the $X\rightarrow A$ electronic transition of BaH to deflect a cryogenic buffer-gas beam and reduce its transverse velocity spread. Multiple experimental methods are employed to characterize the optical cycling dynamics and benchmark theoretical estimates based on rate equation models as well as solutions of the Lindblad master equation for the complete multilevel system. Broader implications for laser cooling and magneto-optical trapping of heavy-metal-containing molecules with narrow transition linewidths are presented. Our results pave the way for producing a new class of ultracold molecules -- alkaline earth monohydrides -- via direct laser cooling and trapping, opening the door to realizing a new method for delivering ultracold hydrogen atoms (Lane 2015 \textit{Phys. Rev. A} 92, 022511).

Optical cycling, radiative deflection and laser cooling of barium monohydride (BaH)

The determination of the matrix elements of single-electron operators for the transitions between electronic states (primarily, the dipole moments of electron transitions) is one of the critical problems in simulating the atomic and molecular spectra. In this study, the efficiency of a simple version of the finite-field method for calculating these values within the Fock-space coupled-cluster theory has been investigated.

Electronic Transition Dipole Moments in Relativistic Coupled-Cluster Theory: the Finite-Field Method

Molecular properties of the thallium monocyanide (Tl·CN) system in its ground electronic state are studied using high-precision ab initio relativistic two-component pseudopotential replacing 60 inner-core electrons of Tl. A relativistic coupled-cluster method with single, double and perturbative triple amplitudes is employed to account for electronic correlations. Extrapolation of results to the complete basis set limit is used for all studied properties. The global potential energy minimum of Tl·CN corresponds to the linear cyanide (TlCN) isomer, while the non-rigid isocyanide-like (TlNC) structure lies by approximately 11 kJ/mol higher in energy. The procedure of restoration of the wavefunction in the “core” region of Tl atom was applied to calculate the interaction of the Tl nuclear Schiff moment with electrons. The parameter X of the interaction of the Tl nuclear Schiff moment with electrons in the linear TlCN molecule equals 7150 a.u. The prospects of using the TlCN molecule for the experimental detection of the nuclear Schiff moment are discussed.

https://www.mdpi.com/2218-2004/7/3/62/pdf

Towards the Search for Thallium Nuclear Schiff Moment in Polyatomic Molecules: Molecular Properties of Thallium Monocyanide (TlCN)

The spin-forbidden a 3 Π − X 1 Σ + , a ′ 3 Σ + − X 1 Σ + and spin-allowed A 1 Π − X 1 Σ + electronic transitions dipole moments of carbon monoxide have been ab initio studied in the framework of multi-reference Fock space coupled cluster method combined with the finite-field approach and the generalized relativistic pseudo-potential model for the effective introducing the relativity in all-electron correlation treatment. Radiative lifetimes, τ i ( v ′ , J ′ ) , of the upper i ∈ [ a 3 Π , A 1 Π ] states were evaluated as a function of vibrational, v ′ , and rotational, J ′ , quantum numbers. The τ a -values derived for the triplet a 3 Π 1 ( 0 , 1 ) and a 3 Π 2 ( 0 , 2 ) states at the highest level of computation were found to be 2.59 and 142 ms, which are near perfect agreement with their best experimental counterparts (J. J. Gilijamse, et al., J. Chem. Phys., 127, 221102 (2007)). The intercombination a − X , a ′ − X transition probabilities were numerically proved to be proportional to α 2 , where α = e 2 / ℏ c is the fine structure constant.

Ab initio relativistic treatment of the a3Π−X1Σ+, a′3Σ+−X1Σ+ and A1Π−X1Σ+ systems of the CO molecule

Nikolai S. Mosyagin, Alexander V. Oleynichenko, Andrei Zaitsevskii, Artur V. Kudrin, Elena A. Pazyuk, and Andrey V. Stolyarov Petersburg Nuclear Physics Institute named by B. P. Konstantinov of National Research Center “Kurchatov Institute”, 188300, Leningradskaya oblast, Gatchina, mkr. Orlova roscha, 1, Russia and Department of Chemistry, Lomonosov Moscow State University, 119991, Moscow, Leninskie gory 1/3, Russia (Dated: December 4, 2021)

A. V. Kudrin

Papers

Electronic Transition Dipole Moments in Relativistic Coupled-Cluster Theory: the Finite-Field Method

Towards the Search for Thallium Nuclear Schiff Moment in Polyatomic Molecules: Molecular Properties of Thallium Monocyanide (TlCN)

Ab initio relativistic treatment of the a3Π−X1Σ+, a′3Σ+−X1Σ+ and A1Π−X1Σ+ systems of the CO molecule

O ct 2 02 0 Ab initio relativistic treatment of the intercombination a 3 Π − X 1 Σ + Cameron system of the CO molecule †