title: Communication: multireference equation of motion coupled cluster: a transform and diagonalize approach to electronic structure.
abstract: The novel multireference equation-of-motion coupled-cluster (MREOM-CC) approaches provide versatile and accurate access to a large number of electronic states. The methods proceed by a sequence of many-body similarity transformations and a subsequent diagonalization of the transformed Hamiltonian over a compact subspace. The transformed Hamiltonian is a connected entity and preserves spin- and spatial symmetry properties of the original Hamiltonian, but is no longer Hermitean. The final diagonalization spaces are defined in terms of a complete active space (CAS) and limited excitations (1h, 1p, 2h, …) out of the CAS. The methods are invariant to rotations of orbitals within their respective subspaces (inactive, active, external). Applications to first row transition metal atoms (Cr, Mn, and Fe) are presented yielding results for up to 524 electronic states (for Cr) with an rms error compared to experiment of about 0.05 eV. The accuracy of the MREOM family of methods is closely related to its favorable extensivity properties as illustrated by calculations on the O2-O2 dimer. The computational costs of the transformation steps in MREOM are comparable to those of closed-shell Coupled Cluster Singles and Doubles (CCSD) approach.
authors.affiliation: Department of Chemistry, University of Waterloo, Waterloo, N2L 3G1, Ontario, Canada.
authors.forename: Marcel
authors.initials: M
authors.lastname: Nooijen
authors.affiliation: J. Heyrovský Institute of Physical Chemistry of AS CR v.v.i., Prague, Czech Republic.
authors.forename: Ondřej
authors.initials: O
authors.lastname: Demel
authors.affiliation: Institut for Physikalische Chemie, Johannes Gutenberg Universität, Mainz, Germany.
authors.forename: Dipayan
authors.initials: D
authors.lastname: Datta
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Liguo
authors.initials: L
authors.lastname: Kong
authors.affiliation: Department of Chemistry, Mohali University, IISER Mohali, Knowledge City, Sector 81, SAS Nagar, Manauli PO 140306, India.
authors.forename: K R
authors.initials: KR
authors.lastname: Shamasundar
authors.affiliation: Quantum Theory Project, University of Florida, Gainesville, Florida 32611-8435, USA.
authors.forename: V
authors.initials: V
authors.lastname: Lotrich
authors.affiliation: Department of Chemistry, University of Waterloo, Waterloo, N2L 3G1, Ontario, Canada.
authors.forename: Lee M
authors.initials: LM
authors.lastname: Huntington
authors.affiliation: Max Planck Institut für Chemische Energiekonversion, Mülheim an der Ruhr, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Neese