Multireference Double Electron Attached Coupled Cluster Method with Full Inclusion of the Connected Triple Excitations: MR-DA-CCSDT

• Published in: Journal of chemical theory and computation Vol. 7; no. 10; pp. 3088 - 3096
• Main Authors: Musiał, Monika, Kucharski, Stanisław A, Bartlett, Rodney J
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 11.10.2011
• Subjects: Quantum Electronic Structure
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/ct200195q

The multireference (MR) double electron attached (DA) coupled cluster (CC) method with full inclusion of the connected triple excitations has been applied to study various kinds of MR situations. The MR-DA-CCSDT (S, Singles; D, Doubles; T, Triples) equations have been derived and implemented in an efficient way with n 6 scaling for the target multireference states. They can be used for producing potential energy curves (PECs) for some classes of molecules, e.g., when double molecular cations separate into two closed shell fragments, illustrated with the example of the Na2 molecule. Correct PECs have also been obtained on dissociation of the N–N and C–C bonds in N2H4 and C2H6 molecules, respectively. Another application is the behavior of the molecular energy when we change dihedral angle in the ethylene molecule: with the MR-DA-CC, we see immediate improvement of the results with smooth, cusp free, behavior around the 90° region.