XChem code

• Represent accurately molecular autoionization and Auger decay beyond the Born-Oppenheimer approximation
• Design interfaces compatible with available ab initio Quantum Chemistry packages for a widespread use in molecular ionization problems
• Keep computational cost at the same level as that required to evaluate bound states at the same level of theory

• The Close Coupling Matrix (CCM) for a user-defined set of ionization channels (each defined as an ionized molecular state coupled to electrons of given angular momenta) and including short range states relevant to the problem at hand
• Scattering states and scattering phases by asymptotic fitting to the analytical solution
• (Partial) photoionization cross sections within perturbation theory
• Lifetime and character of resonances embedded in the molecular continuum, either via analysis of the cross section or via inclusion of a complex absorbing potential in the CCM yielding complex eigen-energies
• The electron dynamics during and after ionization, caused by and probed with ultrashort laser pulses, by solving the time dependent Schrödinger Equation using the CCM
• The angular distribution of photo electrons (MFPADs)

• The theoretical study of ultrafast processes in pump-probe experimental method, such as Attosecond Transient Absorption Spectroscopy (ATAS) and Reconstruction of Attosecond Beatings by Interference of Two-Photon Transitions (RABBITT)
• The investigation of photoionization of complex molecules close to threshold, where electron correlation effects may dramatically affect the (partial) photoionization cross section (Fig. 1)
• The study of ionization processes intrinsically dependent on electron correlation, like Autoionization and Auger Decay
• The computation of potential energy surfaces for the investigation of molecular dynamics during and after fast photoionization events

• Researchers in (computational) quantum chemistry or molecular physics interested in studying electron dynamics in the ionization continuum of molecules (e.g., photoionization, charge migration, etc)
•  Laboratories investigating ultrafast phenomena in many-electron atoms, small and medium size molecular systems

• Gaussian
• and B-Spline hybrid basis sets.

This approach yields the scattering states of the molecular system via the eigenstates of the close coupling matrix (CCM). While useful in their own right, the full potential lies in using the close coupling matrix as a starting point for time dependent calculations. Doing so, one may explicitly model the interaction of molecules with ultrashort (attosecond) pulses. The extreme band widths of such pulses lead to the coherent excitation of multiple ionization channels, whose coupling (accurately described in XCHEM) gives rise to complex phenomena.

An attractive feature of XCHEM is that the architecture of the basis functions (Fig. 2) and the use of Molcas allow one to describe the electronic continuum of medium-size molecules at the same level of theory as multi-configurational SCF methods (CASSCF, RASSCF) do for the ground and the lowest excited states of such molecules. At present the largest systems treated have of the order of ten atoms.

How to download

• Public repository (collaborative environment for developers and contributors): https://gitlab.com/xchem/xchem_public
• Dataverse (all releases and data inked to publications): e-ciencia datos repository
• XChem v1.0.0 (DOI: 10.21950/GHWTML): Dowload source code here

Citing

• Xchem: C. Marante, M. Klinker, I. Corral, J. Gonzalez-Vazquez, L. Argenti, and F. Martin, Hybrid-Basis Close-Coupling Interface to Quantum Chemistry Packages for the Treatment of Ionization Problems, J. Chem. Theory Comput. 2017, 13, 2, 499–514 Publication Date:December 1, 2016.
• GABs basis: C. Marante, L. Argenti, and F. Martın, Hybrid Gaussian–B-spline basis for the electronic continuum: Photoionization of atomic hydrogen, Physical Review A 90, 012506 (2014).
• Code development: XChem v1.0.0 DOI: 10.21950/GHWTML 
• Bibtex:
  @misc{GHWTML_2023,
      title = {},
      author = {González-Vázquez, Jesús and Marante, Carlos and Klinker, Markus and Borràs, Josep V. and Corral, Inés and Argenti,  Luca and Martín, Fernando},
      year = {2023},
      note = {\url{https://doi.org/10.21950/GHWTML}}
  }

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