Use of integral approximations in non-empirical LCAO MO SCF calculations on XCN systems

Abstract

The accuracy of three multicentre integral approximations has been evaluated for a number of LCAO MO SCF calculations on the ground states of HCN and FCN. McLean's results for HCN, using a basis set of best atom atomic orbitals, are best reproduced by two integral approximations suggested by Löwdin, while the Mulliken approximation is poor. All three integral approximations yield total energies lying below the ab initio value. But double-zeta calculations on HCN and FCN, using a fixed CN internuclear separation, both yield minimum total energies which straddle the ab initio values. The calculated equilibrium CH and FC internuclear separations straddle the experimental values of 0.1063 nm and 0.1262 nm, respectively: R_CH= 0.1041 nm and R_FC= 0.1196 nm when using the Mulliken approximation, and R_CH= 0.1104 nm and R_FC= 0.134 nm when using the best Löwdin approximation. Dipole moments and one-electron energies are also calculated for each approximation. The charge distributions are discussed in terms of effective atomic charges and one-electron difference density functions.