  In this work, we combine the many-body formulation of the internally
contracted multireference coupled cluster (ic-MRCC) method with Evangelista's
multireference formulation of the driven similarity renormalization group
(DSRG). The DSRG method can be viewed as a unitary multireference coupled
cluster theory, which renormalizes the amplitudes based on a flow equation
approach to eliminate numerical instabilities. We extend this approach by
demonstrating that the unitary flow equation approach can be adapted for
nonunitary transformations, rationalizing the renormalization of ic-MRCC
amplitudes. We denote the new approach, the renormalized ic-MRCC (ric-MRCC)
method. To achieve high accuracy with a reasonable computational cost, we
introduce a new approximation to the Baker-Campbell-Hausdorff expansion. We
fully consider the linear commutator while approximating the quadratic
commutator, for which we neglect specific contractions involving amplitudes
with active indices. Moreover, we introduce approximate perturbative triples to
obtain the ric-MRCCSD[T] method. We demonstrate the accuracy of our approaches
in comparison to advanced multireference methods for the potential energy
curves of H8, F2, H2O, N2, and Cr2. Additionally, we show that ric-MRCCSD and
ric-MRCSSD[T] match the accuracy of CCSD(T) for evaluating spectroscopic
constants and of full configuration interaction energies for a set of small
molecules.
