COUPLED-CLUSTER CALCULATIONS OF NUCLEAR-MAGNETIC-RESONANCE CHEMICAL-SHIFTS

Theory and implementation of the gauge-including atomic orbital (CIAO) ansatz for the gauge-invariant calculation of nuclear magnetic resona nce chemical shifts are described for the coupled-cluster singles and doubles (CCSD) approach. Results for the shielding constants of the hy drides HF, H2O, NH3, and CH4 as well as for a few multiply bonded syst ems such as CO, N-2, and HCN demonstrate the importance of higher-orde r correlation corrections, as good agreement with experiment is only o btained at the CCSD level and to some extent at partial fourth-order m any-body perturbation theory [SDQ-MBPT(4)] with the latter slightly ov erestimating correlation effects due to single and double excitations. For relative chemical shifts, GIAO-CCSD calculations provide in diffi cult cases (e.g., CO and CF4) more accurate results than previous GIAO -MBPT(2) calculations. But, it seems that it is often more important t o include rovibrational effects (as well as possible molecule-solvent interactions) than higher-order correlation corrections. Despite that, GIAO-CCSD proves to be a powerful tool for the accurate calculation o f NMR chemical shifts. Its capabilities as well as its limitations are demonstrated in shielding calculations for formaldehyde, diazomethane , and ozone. At least for the latter, the description provided by the CCSD ansatz is not sufficient and even higher excitations need to be c onsidered. (C) 1995 American Institute of Physics.