Physics

Electron

Mathematics

Ferromagnetism

Statistics

Mathematical optimization

Full configuration interaction

Wave function

Hamiltonian (control theory)

Exchange interaction

Atomic orbital

Quantum Monte Carlo

Chemistry

Monte Carlo method

Configuration interaction

Molecule

Spin (aerodynamics)

Quantum mechanics

Complete active space

Configuration space

Thermodynamics

Journal of Chemical Theory and Computation

We present a protocol based on unitary transformations of molecular orbitals to reduce the number of nonvanishing coefficients of spin-adapted configuration interaction expansions. Methods that exploit the sparsity of the Hamiltonian matrix and compactness of its eigensolutions, such as the full configuration interaction quantum Monte Carlo (FCIQMC) algorithm in its spin-adapted implementation, are well suited to this protocol. The wave function compression resulting from this approach is particularly attractive for antiferromagnetically coupled polynuclear spin systems, such as transition-metal cubanes in biocatalysis, and Mott and charge-transfer insulators in solid-state physics. Active space configuration interaction calculations on N2 and CN- at various bond lengths, the stretched square N4 compounds, the chromium dimer, and a [Fe2S2]2- model system are presented as a proof-of-concept. For the Cr2 case, large and intermediate bond distances are discussed, showing that the approach is effective in cases where static and dynamic correlations are equally important. The [Fe2S2]2- case shows the general applicability of the method.

Compression of Spin-Adapted Multiconfigurational Wave Functions in Exchange-Coupled Polynuclear Spin Systems