Effective potentials for spectator groups in molecular systems I. Potential curves and binding energies

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A method for representing inactive groups, i.e. spectator groups, in a molecular system by an effective potential is presented. The matrix elements for the spectator's short-range Hartree-Fock potential is stored in an intermediate AO basis, from which it can be transferred into the user basis for the active part of the molecular system. The longer-range of the potential is transferred via a (distributed) multipole expansion. The method is illustrated for the NH3·X (X=NH3, H2O, HF) complexes: binding energies could be reproduced to within 5% by employing the effective NH3 potential (whereby the lone pair was included in the active system), the entire NH3·HF potential curve with a depth of 50 kJ/mol is reproduced within 2 kJ/mol if various intermediate basis sets are chosen. Technical details are discussed; the effective potential can directly be introduced in CI calculations.