The Wuppertal multireference configuration interaction (MRD-CI) program system
Theoretical Chemistry Accounts, 133 (4):
1468--1480 (April 2014)DOI: 10.1007/s00214-014-1468-7
Zusammenfassung
The goal of a full configuration interaction (CI) calculation in a flexible atomic orbital basis can be approached rather closely for small molecular systems by designating a set of reference configurations from which only single- and double-orbital substitutions are allowed to generate the many-electron basis. The present review discusses how configuration-driven algorithms can be applied to form the corresponding Hamiltonian matrix representation from which approximate energy eigenvalues and eigenvectors can be computed. Tables are constructed which simplify the analysis of configuration relationships that determine how a given interaction is calculated. This Table-CI procedure eliminates the need for carrying out lineup permutations of spin orbitals, which was one of the major bottlenecks in early implementations of the multireference CI method. Although this approach is fundamentally variational in character, it can be easily combined with perturbative techniques that allow one to approximate the corresponding full CI energies to a high degree of accuracy. The speed with which Hamiltonian matrix elements can be calculated makes it feasible to use the Direct CI approach which requires that these values must be recalculated in each iteration of the diagonalization procedure, thereby achieving a considerable increase in the overall efficiency of the computations. The effects of spin–orbit coupling and other terms in the relativistic Hamiltonian have been incorporated in a straightforward manner through the use of effective core potentials.