Individualized configuration selection in CI calculations with subsequent energy extrapolation

Abstract

A configuration selection method for CI calculations is discussed and applied in which the energy lowering produced in a secular equation by the addition of a given test species to a series of dominant configurations is used as an ordering parameter. Configurations with energy lowerings below a given energy cut-off value are not included in the final secular equations but instead a method of estimating the combined effect of the neglected species on the corresponding non-selected CI results is developed. The influence of the choice of main configurations used in the selection process is given close examination as well as the importance of the MO basis employed in the treatment as a whole; in the latter case a non-iterative procedure for obtaining approximate natural orbitals for such calculations is suggested. The resulting configuration selection procedure is equally applicable to all types of electronic states in any nuclear geometry and the results of the associated CI calculations are seen to be essentially equivalent to a complete treatment in which all single-and double-excitation species with respect to a series of dominant configurations in a given state are included.

Theoretical predictions of red and near-infrared strongly emitting X-annulated rylenes

“…The DFT/TDDFT calculations are carried out in the TURBOMOLE 6.0 program package. 40 In the semiempirical approach, the molecular vertical excitation energies of the four low-lying excited states at S 0 geometry in oligorylenes are calculated by using multireference configuration interaction with single and double excitations within the modified neglect of differential overlap (MNDO) 41 Hamiltonian (MRCI/MNDO) as implemented by Lei et al 42 and the multireference determinant single and double interactions 43 coupled with the ZINDO Hamiltonian 44 (MRCI/ZINDO), which has been greatly expanded by us in terms of both active space and the number of configurations. For the ab initio side, we employed the second-order perturbation method based on the CASSCF/CASPT2 as implemented in the MOLCAS 6.4 package.…”

“…43,53 Most importantly, comparing to CASPT2, it can be applied to much larger molecules, which offers us an efficient tool for designing light-emitting molecules.…”

A transformed framework for dynamic correlation in multireference problems

“…[21][22][23] Analogues of these methods for multireference problems, such as multireference perturbation theory, [24][25][26][27][28][29] multireference configuration interaction, [30][31][32][33] and multireference coupled cluster and canonical transformation [34][35][36][37][38][39][40][41][42][43][44] theories have also been formulated. However, all of these multireference formulations are algebraically more opaque and computationally much more expensive than their singlereference counterparts.…”

Perspectives from Quantum Chemistry for molecules and nanograins with astrophysical Interest

“…1. The traditional approach (see the article of Dabia Talbi in this volume) is to undertake the direct determination of the eigenvalue(s) and multi-electronic wavefunction(s) (WF) of the molecular system under study [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Even in the case of the ground state, this is a difficult task which can only be reached approximatively.…”