THE CO MOLECULE - THE ROLE OF BASIS-SET AND CORRELATION TREATMENT IN THE CALCULATION OF MOLECULAR-PROPERTIES

Although Hartree-Fock wave functions can provide a semi-quantitative d escription of the electronic structure of molecules, accurate predicti ons cannot be made without explicit inclusion of the effects of electr on correlation. In correlated calculations, the accuracy of the wave f unction is determined by two expansions: the many-electron expansion i n terms of molecular orbitals that defines the form of the wave functi on and the basis set used to expand the one-electron molecular orbital s. Thus, to assess the accuracy of a given wave function (correlation method), it is necessary to examine the dependence of a given property on the basis set. In this work, systematic sequences of correlation c onsistent basis sets ranging in size from double-to sextuple-zeta (cc- pVnZ) have been employed together with several commonly used electron correlation methods, e.g., MPn (n = 2-4), CCSD, CCSD(T), and MRCI, to calculate the spectroscopic constants and selected molecular propertie s of the carbon monoxide molecule. The computed spectroscopic constant s show excellent convergence toward the complete basis set (CBS) limit , and the intrinsic errors of each correlation method have been assess ed and compared. The effects of correlating the is-like core electrons have also been determined using a sequence of core-valence cc-pCVnZ b asis sets with the CCSD(T) and ACPF methods. A number of other propert ies have also been calculated For each correlation method as a functio n of the correlation consistent basis set: the dipole moment, quadrupo le moment, dipole polarizability, and the first and second hyperpolari zabilities. For these calculations, results using the aug-cc-pVnZ basi s sets are compared with those obtained using basis sets incorporating another complete shell of diffuse functions, d-aug-cc-pVnZ. In each c ase well-behaved convergence toward the CBS limit is observed for each theoretical method. For both the calculated spectroscopic constants a nd molecular properties, comparisons are made to previous calculations and the available experimental data. (C) 1997 Elsevier Science B.V.