On the role of dynamic electron correlation in non-orthogonal configuration interaction with fragments

Abstract

Two different approaches have been implemented to include the effect of dynamic electron correlation in the Non-Orthogonal Configuration Interaction for Fragments (NOCI-F) method. The first is based on shifting the...

GronOR: Scalable and Accelerated Nonorthogonal Configuration Interaction for Molecular Fragment Wave Functions

“…More examples of the dependence of the results on the two thresholds described in this and the previous subsection can be found in the main text and the Supporting Information of refs and . On the basis of the experience gained from those test cases, the ones described in the present study, and other (so far unpublished) calculations, the default values of τ MO and τ CI have been set at 10 –5 , which is somewhat on the safe side, especially for τ MO , where a 10 times larger threshold would not lead to noticeable changes in most cases.…”

“…20 Taking the effects of dynamic correlation into account in NOCI-F calculations has been demonstrated to significantly change the electronic coupling in singlet fission studies and to give very accurate results in a study of the magnetic coupling strength in organic radicals. 21 Although the construction of MEBFs from independently optimized but nonorthogonal multiconfiguration fragment wave functions leads to relatively compact wave function expansions in terms of Slater determinants or combinations thereof, the evaluation of the Hamiltonian matrix for such an expansion involves the evaluation of contributions from large numbers of nonorthogonal determinant pairs. These contributions can be evaluated independently, and therefore, a massively parallel procedure emerges naturally.…”

“…Alternatively, the coefficients of the relatively short CAS wave functions can be dressed with the effect of dynamic correlation with the help of intermediate effective Hamiltonian theory . Taking the effects of dynamic correlation into account in NOCI-F calculations has been demonstrated to significantly change the electronic coupling in singlet fission studies and to give very accurate results in a study of the magnetic coupling strength in organic radicals …”

Perturbative Expansion of Nonorthogonal Product Approach for Charge Transfer States

“…The CASSCF/XMCQDPT strategy shows good results for the treatment of the photosynthetic chromophores: bacteriochlorophylls, chlorophylls, and carotenoids . The excitonic couplings calculated based on the CASSCF wave function and excitation energies calculated by CASSCF/XMCQDPT give rather good excitonic Hamiltonians for the modeling of the excitation energy transfer in photosynthesis. , Fragment-based NOCI was in the similar way improved by PT corrections calculated by CASPT2, NEVPT2, and DCD-CAS . However, if the overlap of the wave functions must be taken into account then such approaches as NOPA should be used instead of the excitonic Hamiltonian without overlap contribution (i.e., CASSCF energies at the diagonal and Coulomb couplings at the nondiagonal positions).…”

“…6,9 Fragment-based NOCI was in the similar way improved by PT corrections calculated by CASPT2, NEVPT2, and DCD-CAS. 43 However, if the overlap of the wave functions must be taken into account then such approaches as NOPA should be used instead of the excitonic Hamiltonian without overlap contribution (i.e., CASSCF energies at the diagonal and Coulomb couplings at the nondiagonal positions). The account of the dynamic correlation may not be such straightforward in this case and the overlap of the wave functions may affect the PT correction.…”