Quantum chemical calculations QCISD level

Nowadays ab initio quantum chemical calculations can provide results approaching benchmark accuracy for small molecules in the gas phase [1], and they have proven to be very useful fo complement experimental studies. Small molecules in the gas phase are typically addressed by high-level methods such as CCSD(T), QCISD(T) and MRCI, which in many cases are equally accurate than experiments [2]. A wide variety of properties such as structures [3] thermochemistry [4] spectroscopic quantities [5,6], and kinetics [7] can be effectively computed. 

The final total energy is effectively at the QCISD(T)/6-3114-G(3df,2p) level if the different additivity approximations work well. The validity of these additivity approximations was investigated by performing complete QCISD(T)/6-31l4-G(3df,2p) calculations on the set of 125 test reactions and in most cases the additivity approximations were found to be satisfactory. All of the calculations required for G2 theory are available in the quantum chemical computer program Gaussian 94. ... 

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