Accuracy of the first-order density matrices calculated with approximate coupled-cluster methods including connected triple excitations

Abstract

The magnitudes of errors introduced in the first-order density matrices by approximating the full coupled-cluster single, double, and triple excitation method (CCSDT) by the CCSDT-1a, CCSDT-1b, CCSDT-2, CCSDT-3, CCSDT-4, CCSD + T(CCSD), and CCSD(T) techniques are investigated with the help of the differential density matrix overlap (DDMO) index. From the results of calculations involving the BH, HF, and H 2O molecules at their equilibrium and stretched geometries, it is concluded that the relative errors in the density matrices are always much larger than the corresponding errors in the correlation energies. The CCSDT-4 method usually provides the best level of approximation for both the correlation energies and the first-order density matrices of molecules at their equilibrium geometries. However, for molecules with stretched bonds, none of the schemes appears to be uniformly superior to the others in providing an accurate approximation to the CCSDT density matrices. This implies that none of the aforementioned methods is clearly preferable for calculations of the one-electron properties of molecules at geometries far from equilibrium.