An efficient protocol for computing MCD spectra in a broad frequency range combining resonant and damped CC2 quadratic response theory

Coupled-cluster response theory offers a path to high-accuracy calculations of spectroscopic properties, such as magnetic circular dichroism (MCD). However, divergence or slow convergence issues or are often encountered for electronic transitions in high-energy regions with a high density of states. This is here addressed for MCD by an implementation of damped quadratic response theory for resolution-of-identity coupled cluster singles-and-approximate-doubles (RI-CC2), along with an implementation of the MCD A term from resonant response theory. Combined, damped and resonant response theory provide an efficient strategy to calculate MCD spectra over a broad frequency range and for systems that include highly symmetric molecules with degenerate excited states. The protocol is illustrated by application to Zink Tetrabenzoporphyrin in the energy region 2–8 eV and comparison to experimental data. Timings are reported for the resonant and damped approaches, showing that the greater part of the calculation time is consumed by the construction of the building 1blocks for the final MCD ellipticity. A recommendation on how to utilize the procedure is outlined.