Monte Carlo Configuration Interaction
mcci is a program suitable for generating highly accurate solutions to quantum-many electron problems; there is no inherent limit set on the number of electrons/fermions, but typical calculations to date have considered molecules or clusters with between 10 and 100 electrons. With reasonable sized basis set expansions, > 99% of the correlation energy can be readily obtained for small molecular systems. In principle, there is no limit to the accuracy of the calculations outside of memory and CPU time constraints. Selective convergence on excited states is possible with the program. The program calculates energy matrix elements from one- and two-body integrals, which are read-in externally. The program is independent of the operators used in these expectation values, and hence is equally valid for any spin independent Hamiltonian composed of one- and two-body operators. Accurate calculations of atomic and molecular spectra have been demonstrated with the program. Treatment of spin states is exact within the program.
mcci is simple to use and provides an efficient means for performing configuration interaction (CI) calculations. As with all forms of CI, the basic problem is how to select new configurations for the expansion of the wavefunction and this is the issue addressed by mcci. The mcci program selects configurations through a Monte Carlo generation procedure and then iteratively refines the wavefunction. As a consequence, a compact CI vector results as opposed to traditional approaches such as excitation truncation. The program has been applied to the study of the ground state correlation energy of small molecules, to molecular dissociation problems, and to the calculation of the excited state spectra of atoms. Calculation of the spectra of point defects in solids has been performed, and the electronic states for simplified models of quantum dots and nanowires are also studied with use of the program for electronic applications. The code scales well on parallel architectures.