Excited states of molecules in strong uniform and non-uniform magnetic fields

TL;DR

This paper develops a general Hartree-Fock linear response method with complex orbitals to compute molecular electronic spectra in strong magnetic fields, applicable to various reference states and magnetic field configurations.

Contribution

It introduces a comprehensive implementation of the method for molecules in strong magnetic fields, including non-uniform fields, and compares results with other high-level theories.

Findings

Spectra in uniform fields are qualitatively similar to coupled cluster results.

Non-uniform fields induce spin-forbidden excitations and loss of spin symmetry.

Length and velocity gauges for oscillator strengths are numerically equivalent with complex orbitals.

Abstract

This paper reports an implementation of Hartree-Fock linear response with complex orbitals for computing electronic spectra of molecules in a strong external magnetic fields. The implementation is completely general, allowing for spin-restricted, spin-unrestricted, and general two-component reference states. The method is applied to small molecules placed in strong uniform and non-uniform magnetic fields of astrochemical importance at the Random Phase Approximation level of theory. For uniform fields, where comparison is possible, the spectra are found to be qualitatively similar to those recently obtained with equation of motion coupled cluster theory. We also study the behaviour of spin-forbidden excitations with progressive loss of spin symmetry induced by non-uniform magnetic fields. Finally, the equivalence of length and velocity gauges for oscillator strengths when using complex orbitals is investigated and found to hold numerically.