The chemical shifts observed in nuclear magnetic resonance experiments are the differences in shielding of the nuclear spin in different electronic environments. These are known to depend on intermolecular interactions as evidenced by density-dependent chemical shifts in the gas phase, gas-to-liquid shifts, and adsorption shifts on surfaces. We present the results of the first ab initio intermolecular chemical shielding function calculated for a pair of interacting atoms for a wide range of internuclear separations. We used the localized orbital local origin (LORG) approach of Hansen and Bouman and also investigated the second-order electron correlation contributions using second-order LORG (SOLO). The Ar-39 shielding in Ar2 passes through zero at some very short distance, going through a minimum, and asymptotically approaches zero at larger separations. The Ne-21 shielding function in Ne2 has a similar shape. The Drude model suggests a method of scaling that portion of the shielding function that is weighted most heavily by exp [- V(R)/kT]. The scaling factors, which have been verified in the comparison of Ne-21 in Ne2 against Ar-39 Ar2 ab initio results, allows us to project out from the same Ar-39 in Ar2 ab initio values the appropriate Xe-129 shielding functions in the Xe-Ar, Xe-Kr, and Xe-Xe interacting pairs. These functions lead to temperature-dependent second virial coefficients of chemical shielding which agree with experiments in the gas phase. Ab initio calculations of Ar-39 shielding in clusters of argon are used to model the observed Xe-129 chemical shifts of Xe, Xe2,...,Xe8 trapped in the cages of zeolite NaA.
