0000000000326420
AUTHOR
Miriam Heckert
Basis-set extrapolation techniques for the accurate calculation of molecular equilibrium geometries using coupled-cluster theory
To reduce remaining basis-set errors in the determination of molecular equilibrium geometries, a basis-set extrapolation (BSE) scheme is suggested for the forces used in geometry optimizations. The proposed BSE scheme is based on separating the Hartree-Fock and electron-correlation contributions and uses expressions obtained by straightforward differentiation of well established extrapolation formulas for energies when using basis sets from Dunning's hierarchy of correlation-consistent basis sets. Comparison with reference data obtained at the R12 coupled-cluster level [CCSD(T)-R12] demonstrates that BSE significantly accelerates the convergence to the basis-set limit, thus leading to impro…
Towards a spin-adapted coupled-cluster theory for high-spin open-shell states
A spin-adapted coupled-cluster (SA-CC) scheme based on the additional consideration of spin constraints is proposed for the quantum chemical treatment of high-spin open-shell cases. Its computational feasibility is demonstrated via a pilot implementation within the singles and doubles approximation. Test calculations indicate that the suggested SA-CC scheme provides results of similar accuracy as the more traditional schemes without spin adaptation.
Molecular equilibrium geometries based on coupled-cluster calculations including quadruple excitations
Using analytic gradient techniques and an additivity scheme for the various electron correlation contributions, i.e. core-correlation, contribution due to full treatment of triple excitations and contributions due to quadruple excitations calculated with different basis sets, the accuracy of computed geometrical parameters are analysed in comparison with experiment. For a test set of 12 closed-shell and 5 open-shell molecules, it is found that inclusion of quadruple excitations is essential to reach agreement with experiment. The mean error of 0.002 pm and the standard deviation of 0.040 pm of the present CCSD(T)/cc-pV6Z + core(CCSD(T)/cc-pCVQZ) + T/cc-pVTZ + Q/cc-pVDZ results for the close…