# Combining Density Functional Embedding Theory and DMRG-NEVPT2 to Treat Large Active Spaces: Addressing Electronic Structure Complexity in Single-Atom Alloys

## TL;DR

Researchers combined two advanced computational methods to better understand the electronic structure of single-atom alloys used in catalysis.

## Contribution

A novel combination of DFET/ECW with DMRG-SCF and DMRG-NEVPT2 to handle large active spaces in single-atom alloy studies.

## Key findings

- Conventional multireference methods overbind CO due to incomplete treatment of dopant d-orbitals.
- Larger active spaces with DMRG methods yield more accurate CO adsorption free energies.
- The method is broadly applicable to catalytic reactions on metal surfaces requiring large active spaces.

## Abstract

Single-atom alloys
(SAAs) are an increasingly popular platform
for heterogeneous catalysis because of their distinct electronic structures
and ability to break catalytic linear scaling relationships. This
popularity has led to a proliferation of computational studies probing
SAA reactivity at the density functional theory (DFT) level. However,
some phenomena such as photo- and electrocatalysis require use of
electronic structure methods beyond DFT; such studies are both rare
and fundamentally challenging. Density functional embedding theory
(DFET)/embedded correlated wavefunction (ECW) studies of reactions
on metal surfaces have been shown to provide a reliable way to correct
for DFT-related errors. DFET/ECW studies of chemistry involving SAAs,
however, could require active spaces beyond the capabilities of traditional
multireference methods when transition-metal dopants give rise to
many degenerate states. To overcome this limitation, we combined our
DFET/ECW methodology with the density matrix renormalization group
(DMRG) complete active space self-consistent field (DMRGSCF) and DMRG N-electron valence state second-order perturbation theory
(DMRG-NEVPT2) methods in the PySCF code. Using embedded DMRGSCF and
embedded DMRG-NEVPT2, we analyze CO adsorption on Ni-, Rh-, Pd-, and
Pt-doped Ag(100) with different active spaces. We show that the active
spaces approachable with conventional multireference methods lead
to overbinding of CO due to an inability to treat all of the dopant
d-orbitals on equal footing. Larger active spaces, which are easily
treated by both DMRGSCF and DMRG-NEVPT2, yield much more reasonable
adsorption free energies. Our findings suggest that future multireference
calculations of these systems should similarly employ active spaces
containing all of the dopant d-orbitals along with sp-band orbitals
of the host metal near the Fermi level. Emb-DMRG-NEVPT2 is a method
that can be broadly applied to study catalytic reactions on metal
surfaces when large active spaces are required.