AN AB-INITIO BENCHMARK STUDY OF THE H-]HCO REACTION(CO)

The H+CO-->HCO reaction has been characterized with correlation consis tent basis sets at five levels of theory in order to benchmark the sen sitivities of the barrier height and reaction ergicity to the one-elec tron and n-electron expansions of the electronic wave function. Single and multireference methods are compared and contrasted. The coupled c luster method RCCSD(T) was found to be in very good agreement with Dav idson-corrected internally-contracted multireference configuration int eraction (MRCI+Q). Second-order Moller-Plesset perturbation theory (MP 2) was also employed. The estimated complete basis set (CBS)limits for the the barrier height (in kcal/mol) for the five methods, including harmonic zero-point energy corrections, are MP2, 4.66; RCCSD, 4.78; RC CSD(T), 4.15; MRCI, 5.10; and MRCI+Q, 4.07. Similarly, the estimated C BS limits for the ergicity of the reaction are: MP2, -17.99; RCCSD, -1 3.34; RCCSD(T), -13.79; MRCI, -11.46; and MRCI+Q, -13.70. Additional b asis set explorations for the RCCSD(T) method demonstrate that aug-cc- pVTZ sets, even with some functions removed, are sufficient to reprodu ce the CBS limits to within 0.1-0.3 kcal/mol. (C) 1996 American Instit ute of Physics.