  DIRAC is a freely distributed general-purpose program system for 1-, 2- and
4-component relativistic molecular calculations at the level of Hartree--Fock,
Kohn--Sham (including range-separated theory), multiconfigurational
self-consistent-field, multireference configuration interaction, coupled
cluster and electron propagator theory. At the self-consistent-field level a
highly original scheme, based on quaternion algebra, is implemented for the
treatment of both spatial and time reversal symmetry. DIRAC features a very
general module for the calculation of molecular properties that to a large
extent may be defined by the user and further analyzed through a powerful
visualization module. It allows the inclusion of environmental effects through
three different classes of increasingly sophisticated embedding approaches: the
implicit solvation polarizable continuum model, the explicit polarizable
embedding, and frozen density embedding models. DIRAC was one of the earliest
codes for relativistic molecular calculations and remains a reference in its
field.
