EFFECTS OF ELECTRON CORRELATION IN THE CALCULATION OF NUCLEAR-MAGNETIC-RESONANCE CHEMICAL-SHIFTS

Using second-order many-body perturbation theory [MBPT(2)] and the gauge-including atomic orbital (GIAO) ansatz, electron correlation effects are investigated in the calculation of NMR chemical shieldings and shifts. A thorough discussion of the theory, aspects of the implementation as well as the computational requirements of the GIAO-MBPT(2) method are presented. The performance of the GIAO-MBPT(2) approach is tested in benchmark calculations of C-13, N-15, and O-17 chemical shifts. Comparison with available experimental gas phase NMR data shows that GIAO-MBPT(2) improves in all cases considered here over the GIAO results obtained at the Hartree-Fock self-consistent-field (HF-SCF) level. Correlation effects turn out to be particularly important for molecules with multiple bonds, e.g., carbonyl or cyano compounds, and it seems that GIAO-MBPT(2) slightly overestimates these effects for difficult cases having relatively large correlation contributions of 30 to 110 ppm. For CO, N2, N2O, additional calculations with large basis sets are presented to check the accuracy of the GIAO-MBPT(2) method and the geometry dependence of the calculated chemical shieldings is analyzed.