Exploring the MRCI method for calculating interaction energies: application to the HeNe potential curve

A multi-reference configuration interaction (MRCI) method is described, whi ch is devised for the calculation of interaction energies of van der Waals complexes and applied to calculating the HeNe potential energy curve. The M RCI calculations make use of a generalized Pople-correction in order to acc ount for the lack of size consistency. The orbital space is partitioned int o three subspaces: the first active space (ASI), which contains the strongl y occupied orbitals; the second active space (AS2), which contains the main intra-correlating orbitals; and the external space (ES). It is shown that, to keep the error below +/-0.2 K in the excitation scheme and the active o rbital space it is sufficient to include only sigma-orbitals in AS2 and to use an excitation scheme (labelled Qq-MRCI) that encompasses only up to qua druply excited configurations. The final active orbital space (AS2) turned out to be 2s(He), 2p sigma(He), 3s(Ne), 3p sigma(Ne) and 3d sigma(Ne). Othe r MRCI variants, in which most or all quadruply excited configurations were deleted from the CI expansion (Qt- and Tt-MRCI), were found to be inadequa te. Using the Qq-MRCI scheme together with a 197-orbital 'interaction optim ized' basis set (IO197), the MRCI interaction energy at R = 5.7 a(0) was ca lculated to be -21.12 K. The corresponding values at the MP4 and CCSD(T) le vels of theory are -20.06 K and -20.99 K, respectively, indicating that the MP4 method is inappropriate for highly accurate calculations on this syste m. Fitting the calculated data using a generalized Morse function, includin g an additional C-6/R-6 term to account for a correct long-range behaviour of the potential, the MRCI well depth was calculated to be -21.16 K at R-eq = 5.73 a(0). The MRCI and CCSD(T) potentials have the same quality and are found to be In good agreement with the Hartree-Fock dispersion (HFD-B) pot ential of Keil, M., Danielson, L. J., and Dunlop, P. J., 1991, J. Chem. Phy s., 94, 296. It is concluded that, for basis IO197, the CCSD(T) method is s ufficiently accurate for calculating the HeNe interaction. To recover the s mall, missing contributions (a few tenths of a Kelvin), MRCI should be used .