ACCURATE MULTIREFERENCE CONFIGURATION-INTERACTION CALCULATIONS OF THE POTENTIAL-ENERGY FUNCTION AND THE DISSOCIATION-ENERGY OF N-2

The potential energy function of the N2 molecule is calculated using the internally contracted multireference CI method (CMRCI) and complete active space SCF (CASSCF) reference wave functions. A full CI calculation in a DZP basis set is used to estimate the errors associated with the CMRCI wave function. The dependence of the computed spectroscopic constants and the dissociation energy on the basis set is also investigated. Uncontracted and segmented basis sets are compared with ANO (atomic natural orbital) and other generally contracted basis sets. It is found that the energy optimized "correlation consistent" basis sets of Dunning yield substantially better results than ANO basis sets of the same size. In the largest calculations, which included up to h type basis functions and also accounted for core-core and core-valence correlation effects, the remaining errors are 0.0003 angstrom, 8 cm-1, and 0.7 kcal/mol for r(e), omega-e, and D(e), respectively. The inclusion of an i type basis function reduces the error in the dissociation energy to 0.3 kcal/mol (0.013 eV).