ezDyson is a C++ code that calculates absolute photodetachment/ photoionization cross sections, photoelectron angular distributions (PADs), and anisotropy parameters (beta) using Dyson orbitals computed by an ab initio program. The calculation is based on the following approximations: (1) weak field limit, i.e., the photodetachment or photoionization is treated in a perturbative regime; (2) dipole approximation, i.e., assuming that the wavelength of the radiation field is longer than the size of the molecule and that the process is one-photon (first order in perturbation theory); (3) sudden approximation, i.e., we ignore the interactions between the ejected photoelectron and the remaining core electrons; (4) strong orthogonality condition, i.e., the continuum orbital is orthogonal to all states of initial system. We use plane waves or Coulomb waves to describe the continuum state of the photoelectron.
Dyson orbitals are defined as the overlap between an N electron molecular wavefunction and the N-1 electron wave function of the ionized system:
They contain all the information about the ionized system required for calculating the differential cross-section:
where DkIF is the photoelectron dipole matrix element connecting the initial and the final wave functions:
The photoelectron is described by a plane wave or Coulomb wave decomposed in partial waves basis, |E,l,m>, which contains a radial function, Rkl (spherical Bessel functions or Coulomb Radial function) and the spherical harmonics Ylm.
The code requires information from a quantum chemical calculation (geometry, atomic basis set), details of the experiment (laser polarization, ionization energy, molecular orientation, range of electron kinetic energies), the coefficients of the Dyson orbital in the AO basis, and the norm of the Dyson orbital. The ezDyson input is in xml format. It can be prepared manually, or a Python interface can be used to extract data from an electronic structure program. A Python interface is currently provided only for the Q-Chem outputs. Dyson orbital calculations are implemented within the equation- of-motion coupled-cluster (EOM-CC) suite of methods in Q-Chem. The details of the theoretical model employed as well as benchmark results can be found in the following publications:
 C.M. Oana and A.I. Krylov, J. Chem. Phys. 127, 234106 (2007)
Important: Our early implementation of Dyson orbitals (in the ccman module of Q-Chem) had a bug leading to incorrect norms. The correct implementation is currently in the ccman2 module and was released with Q-Chem version 4.3. We provide several python scripts that work with the ccman2 Dyson code in Q-Chem version 4.3. Consult the ezDyson manual for instructions on how to use these scripts, and the Q-Chem user's manual for EOM-CC Dyson orbital calculations.
Version 4.0 of ezDyson introduces a number of important bug fixes and improvements. The updated manual lists these changes. Please refer to the manual for detailed theoretical background, instructions on running ezDyson, and a description of ezDyson input and output files.
Download ezDyson: ezdyson.v4.tar.gz
Download manual: ezDysonManual.pdf
To install, execute:
A Linux executable is provided. However, the code can be easily recompiled for other platforms by using an appropriate makefile.
The original version of the code was developed by Dr. Melania Oana. Dr. Evgeny Epifanovsky and Dr. Ilya Kaliman have helped to parallelize the code. Dr. Kadir Diri and Dr. Anastasia Gunina contributed to developing the manual. Sahil Gulania wrote a section of how to plot Dyson orbitals. The ccman2 implementation of Dyson orbitals was developed by Anastasia Gunina and Tomek Kuš, with the help of Dr. Evgeny Epifanovsky. Dr. Takatoshi Ichino and Prof. John Stanton helped to validate the implementation. This work has been funded by the National Science Foundation and by the Department of Energy.
To acknowledge ezDyson use the following citation:
"This work was conducted using the resources of the iOpenShell Center for Computational Studies of Electronic Structure and Spectroscopy of Open-Shell and Electronically Excited Species (http://iopenshell.usc.edu).”