ATOMCI: a non-relativistic configuration interaction program for many-electron atoms in LS coupling. Program description and input guide

ATOMCI computes non-relativistic configuration interaction (CI) wave functions and energies of many-electron atoms in the Russell–Saunders (LS) coupling scheme. The matrix elements are obtained from the vacuum expectation values of tensor operators and recoupling coefficients, following Sasaki, and the Hamiltonian matrix is built by a formula-tape method. The program provides Slater- and Gaussian-type basis sets, SCF and restricted MCSCF orbitals, general CI and singles-and-doubles CI from one or several references, perturbative selection of configurations, natural orbitals and an analysis of the CI vector. The method and the program were described in 1974 and in the METECC-94 book; this document accompanies the public release of the program, whose build has been modernized for current Fortran compilers. It describes the structure of the program, its input and output, and the test calculations distributed with it. No new method is presented.