Multireference Perturbation theory, in its different implementations and applied up to second order in the energy, has shown to be a very effective approach for the study of the excited states of small and medium size molecules, with a good computational-cost/accuracy ratio. The agreement with experimental data is in general excellent, with errors often lower than 0.2 eV for the excitation energies. This reassuring result is partially invalidated by the peculiar behaviour of a class of π → π* excited states, called “ionic states”, for which larger errors are found with a marked dependence of the theoretical values on the computational parameters (basis set, active space, partition of the Hamiltonian). Based on the experience gained in the last years in our group [1-4], we present some considerations concerning the nature of these excited states, a first step in the direction to unravel this intricate challenge. The discussion is essentially based on the NEVPT2 [5,6] approach, but it equally applies to all MRPT2 methods. [1] A. Angeli, J. Comp. Chem. 30, 1319 (2009) [2] A. Angeli, R. Improta, F. Santoro, J. Chem. Phys. 130, 174307 (2009) [3] A. Angeli, Int. J. Quant. Chem. 110, 2436 (2010) [4] A. Angeli, M. Pastore, J. Chem. Phys. 134, 184301 (2011) [5] A. Angeli, R. Cimiraglia, S. Evangelisti, T. Leininger, J.-P. Malrieu, J. Chem. Phys. 114, 10252 (2001) [6] A. Angeli, M. Pastore, R. Cimiraglia, Theor. Chem. Acc. 117, 743 (2007)
Multireference perturbation theory: the challenging case of the ionic excited states.
ANGELI, Celestino
2011
Abstract
Multireference Perturbation theory, in its different implementations and applied up to second order in the energy, has shown to be a very effective approach for the study of the excited states of small and medium size molecules, with a good computational-cost/accuracy ratio. The agreement with experimental data is in general excellent, with errors often lower than 0.2 eV for the excitation energies. This reassuring result is partially invalidated by the peculiar behaviour of a class of π → π* excited states, called “ionic states”, for which larger errors are found with a marked dependence of the theoretical values on the computational parameters (basis set, active space, partition of the Hamiltonian). Based on the experience gained in the last years in our group [1-4], we present some considerations concerning the nature of these excited states, a first step in the direction to unravel this intricate challenge. The discussion is essentially based on the NEVPT2 [5,6] approach, but it equally applies to all MRPT2 methods. [1] A. Angeli, J. Comp. Chem. 30, 1319 (2009) [2] A. Angeli, R. Improta, F. Santoro, J. Chem. Phys. 130, 174307 (2009) [3] A. Angeli, Int. J. Quant. Chem. 110, 2436 (2010) [4] A. Angeli, M. Pastore, J. Chem. Phys. 134, 184301 (2011) [5] A. Angeli, R. Cimiraglia, S. Evangelisti, T. Leininger, J.-P. Malrieu, J. Chem. Phys. 114, 10252 (2001) [6] A. Angeli, M. Pastore, R. Cimiraglia, Theor. Chem. Acc. 117, 743 (2007)I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
