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

Using second-order many-body perturbation theory [MBPT(2)] and the gau ge-including atomic orbital (GIAO) ansatz, electron correlation effect s are investigated in the calculation of NMR chemical shieldings and s hifts. A thorough discussion of the theory, aspects of the implementat ion as well as the computational requirements of the GIAO-MBPT(2) meth od are presented. The performance of the GIAO-MBPT(2) approach is test ed in benchmark calculations of C-13, N-15, and O-17 chemical shifts. Comparison with available experimental gas phase NMR data shows that G IAO-MBPT(2) improves in all cases considered here over the GIAO result s obtained at the Hartree-Fock self-consistent-field (HF-SCF) level. C orrelation 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 c ases having relatively large correlation contributions of 30 to 110 pp m. For CO, N2, N2O, additional calculations with large basis sets are presented to check the accuracy of the GIAO-MBPT(2) method and the geo metry dependence of the calculated chemical shieldings is analyzed.