Approximating CCSD(T) nuclear magnetic shielding calculations using composite methods

A series of composite method approximations for the computationally efficient calculation of NMR shieldings has been developed. These approximations utilize basis sets from the pcS-n series, which is shown to converge rapidly toward experimental gas-phase shieldings at CCSD(T). The possibility of using HF, B3LYP, KT3, or MP2 shieldings to approximate results at CCSD(T) was then examined. It was determined that using HF in conjunction with MP2 significantly reduces the CPU time of calculations while having a minimal impact on the accuracy of the predicted shieldings.