title: Efficient and accurate treatment of weak pairs in local CCSD(T) calculations. II. Beyond the ring approximation.
abstract: In order to arrive at linear scaling of the computational cost with molecular size, local coupled cluster methods discriminate pairs of local molecular orbitals according to the spatial separation R of the latter. Only strong pairs are treated at the full coupled cluster level, whereas for weak pairs a lower level of theory (usually Møller-Plesset perturbation theory of second order, MP2) is used. Yet an MP2 treatment of weak pairs is inadequate in certain situations (for example, for describing π-stacking), which calls for an improved but still inexpensive method for dealing with the weak pairs. In a previous contribution, we proposed as a substituent for MP2 the LrCCD3 method, which is based on ring coupled cluster doubles (ring-CCD) and includes all third-order diagrams with energy contributions decaying not quicker than R(-6). In the present work, we explore a still more accurate method, which is based on the same principles. It turned out to be essential to abandon the restriction to ring-CCD, i.e., to include further CCD diagrams beyond the ring approximation. The occurring intermediates turn out to be formally very similar to LMP2 density matrices, such that an efficient evaluation of these non-ring CCD diagrams is possible. Furthermore, a computationally cheap a posteriori estimate for the fourth-order singles contribution to the weak pair energy, which also exhibits a decay behavior of R(-6), is introduced. The resulting method, denoted as LCCD[S]-R(-6), indeed provides a substantial improvement in accuracy over the previous LrCCD3 method at a relatively modest additional computational cost.
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Regensburg D-93040, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Schütz
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Regensburg D-93040, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Masur
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Regensburg D-93040, Germany.
authors.forename: Denis
authors.initials: D
authors.lastname: Usvyat