Department of Chemistry Seminar - Quantum Chemical Approach for Open-Shell Systems based on Broken-Symmetry Method

Speaker: Prof. Yasutaka Kitagawa

Institution: School of Materials Science and Engineering, Nanjing University of Science and Technology

Hosted by: Prof. Haibin SU

Abstract

One of the effective ways to obtain the electronic structures of larger open-shell systems is the broken-symmetry (BS) method. The BS method, such as spin-unrestricted Hartree-Fock (UHF) and spin-unrestricted density functional theory (UDFT), approximately accounts for static correlation corrections by splitting a and b electrons into two different spatial orbitals. Due to its low computational cost, it has been widely used for the theoretical geometry optimization of large spin-polarized systems, such as organic p radicals and polynuclear metal complexes. However, this method always suffers from spin contamination, where higher spin states are mixed into lower spin states. The spin contamination error (SCE) manifests in various properties, such as total energies, optimized structures, and excitation energies, because it stems from the BS wavefunctions themselves.

In the first part of my talk, I will discuss the approximate spin projection (AP) procedure proposed by Yamaguchi based on the Heisenberg Hamiltonian, followed by a geometry optimization method based on the AP approach. In the latter part, I will present some applications of the BS methods, particularly for large-scale open-shell systems.