Ab initio calculations using MPn, QCISD, QCISD(T), and CCSD(T) methodologie
s and cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ correlation-consistrnt basis sets w
ere carried out to determine the most stable structure of the CO2. . .N2O v
an der Waals system. Rovibrational absorption and microwave spectroscopic c
onstants are consistent with an approximately slipped parallel structure, b
ut the experimental data cannot determine the orientation of N2O in the dim
er. MPn and QCI methods lead to contradictory predictions. The QCI most sta
ble structure, with the oxygen atom of N2O nearly over the carbon atom of C
O2, is consistent with the experimental data. (C) 2001 John Wiley & Sons, I
nc.