CALCULATION OF STATIC MOLECULAR-PROPERTIES IN THE FRAMEWORK OF THE UNITARY-GROUP BASED COUPLED-CLUSTER APPROACH

The recently developed and implemented state selective, fully spin-ada pted coupled cluster (CC) method that employs a single, yet effectivel y multiconfigurational, spin-free reference and the formalism of the u nitary group approach (UGA) to the many-electron correlation problem, has been employed to calculate static electric properties of various o pen-shell (OS) systems using the finite field (FF) technique. Starting with the lithium atom, the method was applied at the first-order inte racting space single and double excitation level (CCSD(is)) to several first- and second-row hydrides having OS ground state, namely to the CH, NH, OH, SIH, PH, and SH radicals. In the case of NH we also consid ered three OS excited states. In all cases the dipole moment and polar izability were determined using a high quality basis set and compared with the experiment, whenever available, as well as with various confi guration interaction results and other theoretical results that are av ailable from the literature. The agreement of our CCSD(is) values with experiment is very satisfactory except for the (3) Sigma(-) ground st ate of the NH radical, where the experimentally determined dipole mome nt is too small. No experimental data are available for the correspond ing polarizabilities. It is also shown that the FF technique is not su itable for calculations of higher order static properties, such as the hyperpolarizability beta and gamma tensors. For this reason we formul ate the linear response version of our UGA-based CCSD approach and dis cuss the aspects of its future implementation.