THE ANALYTIC GRADIENT OF THE PERTURBATIVE TRIPLE EXCITATIONS CORRECTION TO THE BRUECKNER DOUBLES METHOD

To determine correlated properties accurately, it is necessary to go beyond the level of single and double excitations from a single determinant and include the effects of the connected triple excitations. A formula for the analytic gradient of the closed-shell perturbative triple excitations correction is presented for the Brueckner doubles variant of coupled cluster theory. Optimised geometries and harmonic frequencies are presented for H2O and the multiple bonded systems HCN, NNO and O3. For these molecules, the use of Brueckner orbitals and the triples contribution show significant effects.