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

Abstract

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.