A bit over a year ago we published our adcc code.
In this work the aim was to develop a toolkit for computational spectroscopy methods
focused on rapid development and interactive hands-on usage
(see the blog article for details).
Our target back then was to simplify method development involving
the algebraic-diagrammatic construction approach (ADC)
to compute excited states energies and properties.
ADC has been a research focus both of myself as well as the group of Andreas Dreuw
and the ADC family of methods have proven in the past to be greatly suited
for describing photochemistry and spectroscopic results.

Employing mainly thread-based parallelism and (apart from our recent inclusion
of libxm) basically no options for swapping stored
tensors to disk, adcc is naturally restricted to problems that fit
into the main memory of a single cluster node. This is fine for developing and
testing new ADC methods, but can be limiting for employing ADC methods
in practice: The code can currently only treat small-sized to medium-sized molecules.

In parallel to adcc we therefore started working on the **Gator project**
in collaboration with the groups of Patrick Norman
and Zilvinas Rinkevicius (both KTH Stockholm).
We now release in a first version.
Apart from an interface to adcc, Gator features a
response library capable of the **complex polarisation propagator (CPP) approach**
for simulating properties such as excited-states polarisabilities
or enabling a direct computation of spectra including broadening.
Additionally it contains a newly developed ADC(2) module
with **MPI-based distributed computing** capabilities.
For this the integral driver of the Veloxchem code from KTH
is used, which allows the ADC(2) computation to be performed in a direct fashion
(i.e. without storing the two-electron-integral tensor).
This makes **ADC(2) simulations** in Gator more memory efficient and allows them
to be **distributed over a few cluster nodes**.
In this publication we provide an overview of Gator's current
capabilities. The full abstract reads

The Gator program has been developed for computational spectroscopy and
calculations of molecular properties using real and complex propagators at the
correlated level of wave function theory. At present, the focus lies on methods
based on the algebraic diagrammatic construction (ADC) scheme up to third-order
of perturbation theory. A Fock matrix-driven implementation of the second-order
ADC method for excitation energies has been realized with an underlying hybrid
MPI/OpenMP parallelization scheme suitable for execution in high-performance
computing cluster environments. With a modular and object-oriented program
structure written in a Python/C++ layered fashion, Gator enables, in addition,
time-efficient prototyping of novel scientific approaches as well as
interactive notebook-driven training of students in quantum chemistry.