Extending the ASS1ST Active Space Selection Scheme to Large Molecules and Excited States

Abstract

Multireference electronic structure methods based on the CAS (complete active space) ansatz are well-established as a means to provide reliable predictions of physical properties of strongly correlated systems. A critical aspect of every CAS calculation is the selection of an adequate active space, in particular as the boundaries for tractable active spaces have been shifted significantly with the emergence of efficient approximations to the Full-CI problem like the density matrix renormalization group and full-CI quantum Monte Carlo. Recently, we proposed an active space selection based on first-order perturbation theory (ASS1ST) that yields satisfactory results for the electronic ground state of a variety of strongly correlated systems. In this work, we present a state-averaged extension of ASS1ST (SA-ASS1ST) that determines suitable active spaces when electronically excited states are targeted. Furthermore, the computational costs of the single state and state-averaged variants are significantly reduced by a simple approximation that avoids the most expensive step of the original method, the evaluation of active space four-electron reduced density matrices, altogether. After the applicability of the approximation is established, test calculations on a biomimetic Mn4O4 cluster demonstrate the enhanced range of ASS1ST in terms of system size and complexity. Furthermore, calculations on [VOCl4]2 –, MeMn­(CO)3-α-diimine, and anthracene show that SA-ASS1ST suggests well-suited active spaces to describe d → d and charge-transfer excitations in transition-metal complexes as well as π → π* excitations in aryl compounds. Finally, the application of ASS1ST on multiple points of the potential energy surface of Cr2 illustrates the applicability of the method even when extremely complicated bonding patterns are met. More importantly, however, it highlights the necessity to use special strategies when different points of a potential energy surface are investigated, e.g., during chemical reactions.

Insight into the X‐ray absorption spectra of Cu‐porphyrazines from electronic structure theory

“…Calculating a wealth of complete multiplets (doublet‐doublet‐ and doublet‐quartet‐type excitations) in the presence of dynamic electron correlation while maintaining a large valence active space that can capture all the essential information regarding the corresponding electronic structure unsurprisingly gives rise to an exhaustive computational cost. Therefore, the aforementioned RASCI approach, together with second‐order n‐electron valence perturbation theory to describe electron correlation (RASCI + NEVPT2), was used in combination with the recently introduced ASS1ST [ 69 ] scheme to choose active spaces that include all strongly correlated orbitals but are as small as possible. [ 55 ] In a nutshell, ASS1ST produces a set of quasirestricted natural orbitals for the internal and external orbital space.…”

Exploring Coupled Cluster Green's function as a method for treating system and environment in Green's function embedding methods

“…For SEET, similar to the active space methods, one crucial challenge is to identify a set of strongly correlated orbitals. For most of the problems, such identification often relies on "chemical intuition", though automatic constructions are also known [21][22][23]. A systematic procedure of increasing the number of active orbitals could essentially alleviate the challenges of the active space choice.…”

The Ranked-Orbital Approach to Selecting Active Spaces

“…The past five years have seen a large amount of research on the topic of automatically selecting the active space. 25,[29][30][31][32][33][34][35][36][37][38][39][40] A new approach for selecting active spaces that has gathered a lot of attention in recent years goes by the name of AutoCAS, 25,30 and is centered around the idea of choosing orbitals that vary in occupation (0, ↑, ↓, 2) within a low-cost or even partially converged DMRG calculation. This variance is measured through the single-orbital entropy, given for an orbital i as 41…”