Multi-reference methods like the complete active space self-consistent field (CASSCF) are used extensively to study electronic structure properties in challenging systems. A majority of these applications however, are limited to studying these properties at geometries optimized at lower levels of theory that may not necessarily capture the correct electronic behavior of the system. The high cost associated with a CASSCF energy and gradient calculation for every point along a trajectory or a reaction coordinate makes obtaining a truly multi-reference potential energy surface prohibitively costly.<br/>The newly developed localized active space self-consistent field (LASSCF) method does not suffer from the same scaling as the CASSCF with respect to the growing size of the active space of the entire system. For molecules with multiple centers that are highly correlated within themselves but only weekly correlated to each other, the LASSCF wave function provides a remarkably good description of the multi-reference nature of the entire molecule. Moreover, analytical LASSCF energy gradients can be directly calculated using the Hellmann-Feynman theorem. This allows us to optimize geometries on the potential energy surfaces for systems of moderate sizes using entirely multireference methods at a much more affordable cost.<br/>This presentation will compare the optimized geometries of stilbene and other a highly conjugated organic molecules that are obtained from LASSCF and CASSCF. Insights into the effects of decoposing and localizing the active space are obtained by comparing different active space decomposition schemes – in particular the effect on the bond lengths and angles connecting two fragments on which different subspaces are localized is studied.

Geometries using localized active space self-consistent field: Effect of fragmentation schemes

ACS Spring Meeting 2021

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