XChem

WHAT IS XChem?

XChem package compiles a set of computational tools able to provide a full quantum mechanical description of molecular ionization in the time domain by including both electronic and nuclear degrees of freedom. The used approach has provided reliable predictions and has opened the way to new applications in emerging scientific disciplines, as attochemistry, single molecule imaging, coherent attosecond control of chemical reactions, etc.

XCHEM is a solution for an all-electron ab-initio calculation of the electronic continuum of molecular systems. XCHEM combines the tools of quantum chemistry (as implemented in Molcas) and scattering theory to accurately account for electron correlation in the single-ionization continuum of atoms, small and medium-size molecules.

The validity of the XChem approach has been demonstrated, among others, by the first observation of

  • The correlated motion of two electrons in helium(1)
  • The ultra-fast charge migration in the amino acid phenylalanine(2)
  • And the temporal built-up of the wave-particle duality in helium(3)

A detailed description of XChem potential applicability in chemistry and biology can be found in a recent review article arising from the XCHEM project.(4)

Krausz and Ivanov Rev. Mod. Phys. 81, 163 (2009).

XChem Applications

It is designed to study molecular photoionization involving multichannel scattering problem.
It is used to

  • 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

What can XCHEM do?

XCHEM can compute:

  • 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)

What can XCHEM be used for?

XCHEM is a valuable tool for:

  • 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

Figure 1: Photoionization cross section of N2 obtained with XCHEM. Left: total cross section compared to experiment (J .Phys. Chem. Lett. 2018, 9, 756-762). Right: Partial cross sections for different symmetries of the molecule before and after ionization and the ejected electron (Phys. Rev. A 98, 033413).

Who is XCHEM for?

XCHEM is a valuable tool for:

  • 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

XChem Approach and Features

XChem, at its core, lies a close coupling expansion combined with the use of

  • 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.

Figure 2: Illustration of the XCHEM basis architecture in benzene. Cyan: B-splines, mangenta: Gaussians at the center of mass of the molecule, blue and black: Gaussians at the atomic sites not overlapping with B-splines.

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.

Relevant Publications

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