The CO molecule: the role of basis set and correlation treatment in the calculation of molecular properties

Abstract

Although Hartree-Fock wave functions can provide a semi-quantitative description of the electronic structure of molecules, accurate predictions cannot be made without explicit inclusion of the effects of electron correlation. In correlated calculations, the accuracy of the wave function is determined by two expansions: the many-electron expansion in terms of molecular orbitals that defines the form of the wave function and the basis set used to expand the one-electron molecular orbitals. 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 consistent 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 properties of the carbon monoxide molecule. The computed spectroscopic constants show excellent convergence toward the complete basis set (CBS) limit, and the intrinsic errors of each correlation method have been assessed and compared. The effects of correlating the ls-like core electrons have also been determined using a sequence of core-valence cc- pCVnZ basis sets with the CCSD(T) and ACPF methods. A number of other properties have also been calculated for each correlation method as a function of the correlation consistent basis set: the dipole moment, quadrupole moment, dipole polarizability, and the first and second hyperpolarizabilities. For these calculations, results using the aug-cc- pVnZ basis sets are compared with those obtained using basis sets incorporating another complete shell of diffuse functions, d-aug-cc- pVnZ. In each case well-behaved convergence toward the CBS limit is observed for each theoretical method. For both the calculated spectroscopic constants and molecular properties, comparisons are made to previous calculations and the available experimental data.