On the accuracy of molecular properties by coupled-cluster methods for some difficult examples: Oxygen atom,iron atom,and cyano radical

Coupled-cluster (CC ) methods at the level of CCSD , CCSD +T (CCSD ), CCSD (T ), CCSDT -1, and CCSDT -3 are applied to calculations of the dipole moment and polarizability of the CN molecule, ionization potentials and electron affinities of the oxygen and iron atoms and CN molecule, and the energy splitting of the ⁵D and ⁵F states of the iron atom. Both UHF and ROHF references are applied. Extended basis sets are used in some comparison of CC data to experiment. All calculated atomic and molecular properties are known as challenging problems, suitable for a careful analysis of the performance of sophisticated versions of the CC approach. Attention is paid to energy terms distinguishing CCSD (T ) from CCSD +T (CCSD ). We exploit results from various iterative and noniterative high-level CC methods in the assessment of error bars in calculations of atomic and molecular properties. © 1994 John Wiley & Sons, Inc.