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A new internally contracted MRCI code: MRCIC

A new internally contracted MRCI code [see K. R. Shamasundar, G. Knizia, and H.-J. Werner, A new internally contracted multi-reference configuration interaction (MRCI) method, J. Chem. Phys. 135, 054101 (2011)] is now available. As compared to the old MRCI code it has the following advantages: (i) inactive orbitals (correlated in MRCI but closed-shell in the reference function) are treated explicitly, i.e., no density matrices and coupling coefficients need to be computed that involve these orbitals. Thus, in principle any number of inactive orbitals can be correlated, without the previously existing limitation to 32 correlated orbitals. Furthermore, additional configuration spaces are internally contracted (as in the RS2C code), resulting in a much improved efficiency, particularly for cases with many inactive orbitals. Currently, the method is implemented only for single-state calculations, but an extension to multi-state cases is under development and will be provided in the near future. 2017-12-11