Ultracold polar molecules are crucial components in a wide range of cross-disciplinary experiments, including controlled chemistry, quantum simulation, and precision measurements. Thus, the design and interpretation of such experiments require detailed knowledge of molecular properties. Many of these properties can be predicted using modern _ab initio_ electronic structure methods, which I will demonstrate on a few examples.

In the first part, I will discuss high-accuracy predictions for two diatomic molecules: NaLi in the a³Σ⁺ state [1] and LiCr in the a⁸Σ⁺ state [2]. In both cases, we employ a hierarchy of coupled-cluster wavefunctions and extended Gaussian basis sets. Additionally, we account for nonadiabatic, relativistic, and quantum electrodynamic (QED) effects. The resulting potentials enable reliable predictions of ultracold scattering properties in complex many-electron systems directly from first principles.

In the second part, I address the properties of intermediate triatomic complexes formed during nonreactive collisions between an ultracold alkali-metal molecule and an alkali-metal atom. For the KRb (X¹Σ⁺) + Rb(2S) system [3], we identify an energetically accessible conical intersection between the ground and first excited electronic states, accompanied by an enhancement of spin-rotation coupling. This interaction may be involved in the experimentally observed hyperfine-to-rotational energy transfer. In the NaLi(a³Σ⁺) + Na(2S) system [4, 5], nonadditive three-body interactions reshape the potential energy surface. The combined effects of electron spin-spin and spin-rotation interactions, together with potential anisotropy, alter the collision dynamics. Together, these results demonstrated the intrinsic complexity of ultracold atom-molecule collisions, which involve vibrational, rotational, and spin degrees of freedom.

[1] Gronowski, M., Koza, A. M., and Tomza, M., Ab initio properties of the NaLi molecule in the  electronic state, Physical Review A 102, 020801 (2020)

[2] Finelli, S., Ciamei, A., Restivo, B., Schemmer, M., Cosco, A., Inguscio, M., Trenkwalder, A., Zaremba-Kopczyk, K., Gronowski, M., Tomza, M., and Zaccanti, M., Ultracold LiCr: A New Pathway to Quantum Gases of Paramagnetic Polar Molecules, PRX Quantum 5, 020358 (2024)

[3] Liu, Y.-X., Zhu, L., Luke, J., Babin, M. C., Gronowski, M., Ladjimi, H., Tomza, M., Bohn, J. L., Tscherbul, T. V., and Ni, K.-K., Hyperfine-to-rotational energy transfer in ultracold atom-molecule collisions of Rb and KRb, Nature Chemistry 17, 688-694 (2025)

[4] Park, J. J., Son, H., Lu, Y.-K., Karman, T., Gronowski, M., Tomza, M., Jamison, A. O., and Ketterle, W., Spectrum of Feshbach Resonances in NaLi + Na Collisions, Physical Review X 13, 031018 (2023)

[5] Karman, T., Gronowski, M., Tomza, M., Park, J. J., Son, H., Lu, Y.-K., Jamison, A. O., and Ketterle, W., Ab initio calculation of the spectrum of Feshbach resonances in NaLi+Na collisions, Physical Review A 108, 023309 (2023)