Localized Active Space Pair-Density Functional Theory

Abstract

Accurate quantum chemical methods for the prediction of spin-state energy gaps for strongly correlated systems are computationally expensive and scale poorly with the size of the system. This makes calculations for many experimentally interesting molecules impractical even with abundant computational resources. In previous work, we have shown that the localized active space (LAS) self-consistent field (SCF) method is an efficient way to obtain multi-configuration SCF wave functions of comparable quality to the corresponding complete active space (CAS) ones. To obtain quantitative results, a post-SCF method is needed to estimate the complete correlation energy. One such method is multiconfiguration pair-density functional theory (PDFT), which calculates the energy based on the density and on-top pair density obtained from a multiconfiguration wave function. In this work we introduce localized-active-space pair-density functional theory, which uses a LAS wave function for subsequent PDFT calculations. The method is tested for computing spin-state energy gaps in conjugated organic molecules and bimetallic compounds and is shown to give results within 0.05 eV of the corresponding CAS-PDFT results at a significantly lower cost. File list (2) download file view on ChemRxiv Localized_active_space_pair_density_functiona_theory... (594.23 KiB) download file view on ChemRxiv Localized_active_space_pair_density_functiona_theory.pdf (2.40 MiB)

Localized Active Space State Interaction: A Multireference Method For Chemical Insight

“…In our previous, work we have used this flexibility to change S and M S for specific subspaces to model ground and excited spin states of the molecule. 21,38 For example, the ferromagnetic and anti-ferromagnetic coupling of locally high-spin (quintet) Fe [II] 21 This requires the generation of symmetry adapted molecular wave functions as linear combinations of multiple LAS states.…”

Localized Quantum Chemistry on Quantum Computers

“…On the theoretical side, one strategy is to identify subspaces of the CAS that can be treated on different footings 26,27 or interact with one another only weakly [28][29][30][31][32] . The localized active-space self-consistent field (LASSCF) method [33][34][35][36] , also known as the cluster mean-field (cMF) method, 37 is an example of such a strategy. LASSCF is designed for applications in which electrons are strongly correlated in different weakly interacting physical regions of a molecule and approximates the strongly correlated part of the wave function as a single antisymmetrized product of subspace wave functions.…”

“…Some of the authors have recently shown that LASSCF accurately reproduces the CASSCF spin-state energy gaps of bimetallic compounds and the simultaneous dissociation of two double bonds in bisdiazene at a significantly reduced cost 34,35 . However, LASSCF fails to recover any electron correlation between fragments, for example in the cis-trans isomerization of stilbene and similar systems 36 . Moreover, methods to restore the missing correlation variationally 38 , perturbatively 37,39 , or via the coupled-cluster (CC) approach 40 on classical computers must usually enumerate a general many-body basis for each fragment.…”

Localized Quantum Chemistry on Quantum Computers

“…On the theoretical side, one strategy is to identify subspaces of the CAS that can be treated on different footings 26,27 or interact with one another only weakly [28][29][30][31][32] . The localized active-space self-consistent field method (LASSCF) is an example of such a strategy [33][34][35][36] . LASSCF is designed for applications in which electrons are strongly correlated in different weakly interacting physical regions of a molecule and approximates the strongly correlated part of the wave function as a single antisymmetrized product of subspace wave functions.…”

“…Some of the authors have recently shown that LASSCF accurately reproduces the CASSCF spin-state energy gaps of bimetallic compounds and the simultaneous dissociation of two double bonds in bisdiazene at a significantly reduced cost 34,35 . However, LASSCF fails to recover any electron correlation between fragments, for example in the cis-trans isomerization of stilbene and similar systems 36 . Moreover, because the wave function of each fragment is a general manybody wave function, it is not straightforward 10,37 to apply traditional perturbative or truncated coupled-cluster (CC) corrections based on second quantization 38,39 on top of a LASSCF reference to recover the missing correlation, at least on classical computational hardware.…”