CCSD(T) EXPECTATION VALUE CALCULATIONS OF FIRST-ORDER PROPERTIES

An expectation value approach to calculations of first-order propertie s using the non-iterative, triple-excitation amplitudes in the coupled cluster wave function is exploited. Three methods are suggested and a nalysed using the many body perturbation theory (MBPT) expansion argum ents. The first method, in which non-iterative triple-excitation ampli tudes are used in the expression for the expectation values, makes the wave function accurate through the second order of MBPT. In the secon d method, which is an extension of the first, effects of triple-excita tion amplitudes are coupled with single-and double-excitation amplitud es. The correlated density matrix equivalent through the fourth order to that obtained when CCSDT-la amplitudes are used is employed in the third method. The suggested methods are tested on dipole moment and po larizability calculations for several diatomic closed-shell molecules and are compared to other related approaches.