XMOLECULE is a computational toolkit for simulating electronic and nuclear dynamics induced by x-rays. Electrons are treated quantum mechanically and nuclear motions are solved classically. Based on the Hartree-Fock-Slater method, XMOLECULE solves the Schrödinger equations for calculating electronic structure of multiple-core-hole states that may be formed during interaction with intense x-ray pulses. To achieve an efficient way of calculating molecular core-hole states, it employs numerical atomic orbitals as basis functions, which are optimized for respective atomic core-hole states [1]. For rate and cross section calculations, it utilizes atomic continuum states [2]. In other words, XMOLECULE takes full advantages of XATOM to perform electronic structure and ionization dynamics calculations. During time evolution, xmolecule can handle electronic dynamics among highly excited states created by x-rays, hopping back and forth between potential energy surfaces separated by a few hundreds eV and keV. For nuclear dynamics, XMOLECULE solves the Newton equations based on molecular gradients calculated on-the-fly. These electronic dynamics and nuclear dynamics are coupled to each other and treated together with different time scales via a Monte Carlo on-the-fly approach.

XMOLECULE was applied to quantitatively describe the XFEL–molecule interaction for the first time. We presented a joint experimental and theoretical study of small polyatomic molecules irradiated by ultra-intense hard X-rays, capturing a detailed picture of ultrafast ionization and fragmentation dynamics of molecules [3]. The unprecedentedly high-intensity x-rays knock out lots of electrons from a molecule, creating a sort of molecular black hole as depicted in the figure. It sucks electrons away from neighboring atoms and ejects them furthermore, resulting in significant ionization enhancement for molecules. We named this new phenomenon as CREXIM (charge-rearrangement-enhanced x-ray ionization of molecules) and it provides new insights for understanding the process of radiation damage at high x-ray intensity and designing future XFEL experiments. This story of molecular black hole has been highlighted by the media as the followings:

• DESY News: X-ray pulses create “molecular black hole”
• Newsweek: ‘Black hole’ created by strongest ever x-ray laser
• C&EN: X-rays induce electron-gobbling ‘black holes’
• IBTimes: ‘Black hole’ created in lab with X-ray laser – and it's stronger than the type you find in space
• Scientific American: X-ray lasers make atoms act like “black holes” in molecules
• Dong-A Science 동아사이언스: 아름다운 빛의 춤사위, ‘분자 블랙홀’ [과학동아 2017년 7월호]

1. International Workshop on Intense-field Short-wavelength Atomic and Molecular Processes (Hamburg, Germany, July 18-20, 2015) [invited talk] [BibTeX][slide: 4Mb][abstract][link]
2. 4th CFEL Symposium 2015 (CFEL, Timmendorfer Strand, Germany, September 30-October 2, 2015) [oral presentation] [BibTeX]
3. The 47th Meeting of the Division of Atomic, Molecular and Optical Physics (American Physics Society, Providence, Rhode Island, United States, May 23-27, 2016) [invited talk] [BibTeX][slide: 4Mb][abstract][link]
4. The electronic-structure problem in theoretical strong-field physics (ITAMP, Harvard University, Cambridge, Massachusetts, United States, October 10-14, 2016) [invited talk] [BibTeX][slide: 12Mb][link]
5. Korean Physical Society Fall Meeting (Korean Physical Society, Gwangju, Korea, October 19-21, 2016) [invited talk] [BibTeX][slide: 5Mb][abstract][link]
6. Workshop on Science of “From Matter to Materials and Life” (Hamburg, Germany, December 14-16, 2016) [oral presentation] [BibTeX][abstract][link]
7. New trends in theory for experiments at advanced light sources (European XFEL and CFEL, Schenefeld and Hamburg, Germany, November 29-December 1, 2017) [invited talk] [BibTeX][link]
8. The 7th Topical Conference of the Indian Society of Atomic and Molecular Physics (Indian Society of Atomic and Molecular Physics, Tirupati, India, January 6-8, 2018) [invited talk] [BibTeX][pdf][slide: 8Mb][abstract][link]
9. Light-Matter Interaction: Recent Advances in Theory (DESY, Hamburg, Germany, January 25, 2018) in Satellite Workshop of European XFEL and DESY Photon Science Users' Meeting 2018 [invited talk] [BibTeX][slide: 7Mb][link]
10. Attosecond Physics at the Nanoscale (Center for Theoretical Physics of Complex Systems, Institute for Basic Science, Daejeon, Korea, October 29-November 2, 2018) [invited talk] [BibTeX][slide: 5Mb][abstract][link]
11. 28th Annual International Laser Physics Workshop (Gyeongju, Korea, July 8-12, 2019) [invited talk] [BibTeX][slide: 3Mb][abstract][link]

1. Seminar on Dynamical Aspects of Theoretical Chemistry (Department of Chemistry, Free University of Berlin, Berlin, Germany, November 19, 2015) [invited talk] [BibTeX][slide: 9Mb][abstract]
2. Physical Colloquium (Institute of Physics, University of Kassel, Kassel, Germany, December 3, 2015) [invited talk] [BibTeX][pdf][slide: 9Mb][abstract]
3. Physics and Photon Science Colloquium (Department of Physics and Photon Science, GIST, Gwangju, Korea, October 18, 2016) [invited talk] [BibTeX][pdf][slide: 11Mb][abstract][link]
4. European XFEL Theory Seminar (European XFEL, Schenefeld, Germany, June 22, 2017) [invited talk] [BibTeX][pdf][slide: 10Mb][abstract][link]
5. Special seminar (Department of Chemistry, KAIST, Daejeon, Korea, October 31, 2018) [oral presentation] [BibTeX][slide: 12Mb][abstract][link]
6. Physics/BK21+ Seminar (Department of Physics, POSTECH, Pohang, Korea, July 15, 2019) [invited talk] [BibTeX][slide: 11Mb][abstract][link]
7. Physics Seminar (Department of Physics, UNIST, Ulsan, Korea, July 16, 2019) [invited talk] [BibTeX][slide: 10Mb][abstract]

1. Multiphoton Processes (Gordon Research Conference, Bryant University, Rhode Island, US, June 24-29, 2018) [poster] [BibTeX][abstract][link]
2. European XFEL and DESY Photon Science Users' Meeting (DESY, Hamburg, Germany, January 23-25, 2019) [poster] [BibTeX][poster: 7Mb][abstract][link]