Pade extrapolated effective Hamiltonians in the Fock space relativistic coupled cluster method

摘要

The application of the conventional multiroot multireference coupled cluster methods to the description of molecular excited electronic states in wide areas of molecular geometries are frequently prevented by numerical instabilities due to the presence of intruder states. An efficient solution of this problem can consist in the intruder state avoidance through appropriate modification of energy denominators in coupled cluster equations followed by the suppression of the errors caused by this modification by the operator (matrix) Pade extrapolation of the sequences of resulting approximate effective Hamiltonians. Simple numerical illustrations of the efficiency of the extrapolation procedure are provided on examples of implementations within the Fock space relativistic coupled cluster approach.