Improved description of the orbital relaxation effect by practical use of the self-energy

Abstract

The self-energy shift in the orbital relaxation (OR) term of the polarization propagator complete through the second-order is presented. In combination with the optimal damping parameter in the OR term, the modified propagator produces the excitation energy of the coupled-cluster with singles and doubles (CCSD) accuracy. The self-energy shift requires the floating-point operation of O(N4), where N refers to the magnitude of the molecular size. Because the second-order polarization propagator requires the floating-point operation of O(N5), the additional O(N4) computational effort to construct the self-energy is negligibly small. Numerical results are shown for several molecules including glycine, 2,3,5,6-tetrafluorobenzene, and naphthalene, and promising agreements with those of CCSD are confirmed within less than 0.2 eV. The basis set dependence is also tested for the water molecule using aug-cc-pV NZ (N = D-7), where this newly developed approach mimics the behavior of the CCSD values. The self-energy shifting for the second-order response matrix in combination with the use of a dumping parameter is efficiently implemented for calculations of medium-sized molecular systems, including glycine and naphthalene. The developed approach provides CCSD-like accuracy at a more affordable computational expense. © 2014 Wiley Periodicals, Inc.