DIRAC is a freely distributed general-purpose program system for one-, two-, and four-component relativistic molecular calculations at the level of Hartree–Fock, Kohn–Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, electron propagator, and various flavors of coupled cluster 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 for the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding model, and the frozen density embedding model.
The generic acronym X2C (pronounced as “ecstacy”) for exact two-component Hamiltonians resulted from intensive discussions among H. J. Aa. Jensen, W. Kutzelnigg, W. Liu, T. Saue, and L. Visscher during the Twelfth International Conference on the Applications of Density Functional Theory (DFT-2007), Amsterdam, 26–30 August 2007. Note that the “exact” here means only that all the solutions of the Dirac-based Hamiltonian can be reproduced up to machine accuracy. It is particularly meaningful when compared with finite order quasirelativistic theories.