title: Conical intersections in triplet excited states of methylene from the anti-Hermitian contracted Schrödinger equation.
abstract: A conical intersection in triplet excited states of methylene is computed through the direct calculation of two-electron reduced density matrices (2-RDMs) from solutions of the anti-Hermitian contracted Schrodinger equation (ACSE). The study synthesizes recent extensions of the ACSE method for the treatment of excited states [G. Gidofalvi and D. A. Mazziotti, Phys. Rev. A 80, 022507 (2009)] and arbitrary-spin states [A. E. Rothman, J. J. Foley, and D. A. Mazziotti, Phys. Rev. A 80, 052508 (2009)]. We compute absolute energies of the 1 (3)B(1), 1 (3)A(2), and 2 (3)B(1) states of methylene (CH(2)) and the location of the conical intersection along the 1 (3)A(2)-2 (3)B(1) potential-energy surfaces. To treat multireference correlation, we seed the ACSE with an initial 2-RDM from a multiconfiguration self-consistent field (MCSCF) calculation. The ACSE produces energies that significantly improve upon those from MCSCF and second-order multireference many-body perturbation theory, and the 2-RDMs from the ACSE nearly satisfy necessary N-representability conditions. Comparison of the results from augmented double-zeta and triple-zeta basis sets demonstrates the importance of augmented (or diffuse) functions for determining the location of the conical intersection.
authors.affiliation: Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: James W
authors.initials: JW
authors.lastname: Snyder
authors.forename: Adam E
authors.initials: AE
authors.lastname: Rothman
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Foley
authors.forename: David A
authors.initials: DA
authors.lastname: Mazziotti
mesh_terms: D000465:Algorithms; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D011789:Quantum Theory
chemical_list: C504223:methylene radical; D008697:Methane