*ITAMP Lunch Seminar* *Speaker: *Mariusz Pawlak (Nicolaus Copernicus University) *Date:* Thursday, March 15 *Time:* 12:00-1:00 pm Includes Pizza. *Title: *Adiabatic variational theory for anisotropic atom−diatom collisions at very low temperatures *Abstract: *We recently developed an adiabatic variational theory for low-energy collisions between atoms and diatomic molecules [1,2]. Our approach reduces the dimensionality of the Hamiltonian under consideration without losing essential physics and significantly enhances the computational efficiency. The five degrees of freedom problem, where the bond length in the diatomic molecule is kept fixed, is replaced with a set of uncoupled one-dimensional subproblems. As an illustrative numerical example, the theory has been applied to calculate the Penning ionization reaction rate of the excited metastable helium atom and the molecular hydrogen for both configurations: para and ortho. The results are in very good agreement with the latest experimental findings [3]. [1] M. Pawlak, Y. Shagam, E. Narevicius, N. Moiseyev, J. Chem. Phys. 143, 074114 (2015). [2] M. Pawlak, Y. Shagam, A. Klein, E. Narevicius, N. Moiseyev, J. Phys. Chem. A 121, 2194 (2017). [3] A. Klein, Y. Shagam, W. Skomorowski, P. S. Zuchowski, M. Pawlak, L. M. C. Janssen, N. Moiseyev, S. Y. T. van de Meerakker, A. van der Avoird, C. P. Koch, E. Narevicius, Nature Phys. 13, 35 (2017). *Location: *B-106 @ Center for Astrophysics (60 Garden Street) *Directions: *After entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, B-106 will be at the end of the hallway on the left side.