Configuration interaction (CI) is one of the most general ways to improve upon Hartree-Fock theory by adding a description of the correlations between electron motions. Simply put, a CI wavefunction is a linear combination of Slater determinants (or spin-adapted configuration state functions), with the linear coefficients being determined variationally via diagonalization of the Hamiltonian in the given subspace of determinants. The simplest standard CI method which improves upon Hartree-Fock is a CI which adds all singly and doubly substituted determinants (CISD). The CISD wavefunction has fallen out of favor because truncated CI wavefunctions short of full configuration interaction are not size-extensive, meaning that their quality degrades for larger molecules. MP2 offers a less expensive alternative whose quality does not degrade for larger molecules and which gives similar results to CISD for well-behaved molecules. CCSD is usually a more accurate alternative, at only slightly higher cost.
For the reasons stated above, the CI code in PSI3 is not optimized for CISD computations. Instead, emphasis has been placed on developing a very efficient program to handle more general CI wavefunctions which may be helpful in more challenging cases such as highly strained molecules or bond breaking reactions. The detci program is a fast, determinant-based CI program based upon the string formalism of Handy [1]. It can solve for restricted active space configuration interaction (RAS CI) wavefunctions as described by Olsen, Roos, Jorgensen, and Aa. Jensen [2]. Excitation-class selected multi-reference CI wavefunctions, such as second-order CI, can be formulated as RAS CI's. A RAS CI selects determinants for the model space as those which have no more than n holes in the lowest set of orbitals (called RAS I) and no more than m electrons in the highest set of orbitals (called RAS III). An intermediate set of orbitals, if present (RAS II), has no restrictions placed upon it. All determinants satisfying these rules are included in the CI.
The detci program is also very efficient at full configuration interaction wavefunctions, and is used in this capacity in the complete-active-space self-consistent-field (CASSCF) code. Use of detci for CASSCF wavefunctions is described in the following section of this manual.
As just mentioned, the PSI3 program is designed for challenging chemical systems for which simple CISD is not suitable. Because CI wavefunctions which go beyond CISD (such as RAS CI) are fairly complex, typically the detci program will be used in cases where the tradeoffs between computational expense and completeness of the model space are nontrivial. Hence, the user is advised to develop a good working knowledge of multi-reference and RAS CI methods before attempting to use the program for a production-level project. This user's manual will provide only an elementary introduction to the most important keywords. Additional information is available in the man pages for detci.
The division of the molecular orbitals into various subspaces such as RAS spaces, or frozen vs active orbitals, etc, needs to be clear not only to the detci program, but also at least to the transformation program (and in the case of MCSCF, to other programs as well). Thus, orbital subspace keywords such as RAS1, RAS2, RAS3, frozen_docc, frozen_uocc, active, etc., need to be in the psi:() or default:() sections of input so they may also be read by other modules.