Analytic first and second derivatives for the CCSDT-n (n=1-3) models: a first step towards the efficient calculation of CCSDT properties

Analytic first and second derivatives of the energy are implemented for closed-shell systems described by the CCSDT-n (n = 1, 1b, 2 and 3) and CC3 electron correlation models. A detailed discussion of the computational requirements of these calculations is given, along with diagrammatic formulas for all relevant quantities. The method is applied to calculate the nuclear magnetic shielding of H2O, CO and N2O and the structure and properties of propadienylidene.