Atomic clocks are the most accurate time measurement device available to date. These clocks use the frequency of atomic transition between two atomic states as the timekeeping parameter [1]. The accuracy of an atomic clock is defined in terms of the fractional frequency shift Δf/f0, where f0 is the frequency of atomic transition of an isolated atom and Δf is the change in f0 due to environmental perturbations such as electric and magnetic fields, black-body radiation, relativistic effects, etc. Accurate atomic structure calculations, by providing accurate estimates of the various frequency shifts, play a crucial role in predicting the accuracy of an atomic clock. Such theoretical predictions provide important inputs and guide the atomic clock experiments. We have developed a relativistic coupled-cluster (RCC) theory-based method (and code) to accurately predict atomic clock properties [2]. We have successfully developed and implemented RCC methods for atomic clock properties and other spectroscopic properties calculations in closed-shell, one-valence, and two-valence atomic systems [3-6]. In this talk, I will discuss these RCC methods and their application to various atoms and ions, as a part of my PhD thesis work. In the second part of my presentation, I will briefly discuss my current postdoctoral research and share some of my recent contributions to the development of the DFT-based CP2K molecular dynamics package. Molecular spectroscopy such as Raman, Infrared (IR), and Vibrational Circular Dichroism (VCD) are powerful tools for characterizing molecules and materials. The analytic calculation of IR and VCD spectra using molecular orbital (MO)-based response theory has recently been implemented in CP2K [7,8]. However, MO-based methods are known to have cubic scaling with system size. To address this limitation, we have implemented and computed these response properties using a linear-scaling atomic orbital (AO)-based linear response theory employing an optimization scheme introduced by T. Helgaker et al. [9,10]. This approach improves upon previous MO-based algorithms, as AO-based calculations exhibit linear scaling. The IR and VCD spectra computed using the AO-based response solver are consistent with those obtained from MO-based methods.

1. A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik, and P. O. Schmidt, Rev. Mod. Phys. 87, 637 (2015).
2. B. K. Mani and D. Angom, Phys. Rev. A 83, 012501 (2011).
3. R. Kumar, S. Chattopadhyay, B. K. Mani, and D. Angom, Phys. Rev. A 101, 012503 (2020).
4. R. Kumar, S. Chattopadhyay, D. Angom, and B. K. Mani, Phys. Rev. A 103, 022801 (2021).
5. R. Kumar, S. Chattopadhyay, D. Angom, and B. K. Mani, Phys. Rev. A 103, 062803 (2021).
6. R. Kumar, D. Angom, and B. K. Mani, Phys. Rev. A 106, 032801 (2022).
7. E. Ditler, C. Kumar, and S. Luber, The Journal of Chemical Physics 154, 104121 (2021).
8. E. Ditler, T. Zimmermann, C. Kumar, and S. Luber, Journal of Chemical Theory and Computation 18, 2448 (2022).
9. T. Helgaker, H. Larsen, J. Olsen, and P. Jørgensen, Chemical Physics Letters 327, 397 (2000).
10. H. Larsen, T. Helgaker, J. Olsen, and P. Jørgensen, The Journal of Chemical Physics 115, 10344 (2001).