A multistate local coupled cluster CC2 response method based on theLaplace transform

摘要

A new Laplace transform based multistate local CC2 response method for calculating excitationenergies of extended molecular systems is presented. By virtue of the Laplace transform trick, theeigenvalue problem involving the local CC2 Jacobian is partitioned along the doubles-doubles block(which is diagonal in the parental canonical method) without losing the sparsity in the integral,amplitude, and amplitude response supermatrices. Hence, only an effective eigenvalue probleminvolving singles vectors has to be solved, while the doubles part can be computed on-the-fly.Within this framework, a multistate treatment of excited states with state specific and adaptive localapproximations imposed on the doubles part is straightforwardly possible. Furthermore, in thecontext of the density fitting approximation of the two-electron integrals, a procedure to specify thelocal approximation, i.e., the restricted pair lists and domains, on the basis of an analysis of theobject to be approximated itself is proposed. Performance and accuracy of the new Laplacetransformed density fitted local CC2 (LT-DF-LCC2) response method are tested for set of differenttest molecules and states. It turns out that LT-DF-LCC2 response is much more robust than theearlier local CC2 response method proposed before, which failed to find some excited states indifficult cases.