Excited-state methods for molecular systems: Performance, pitfalls, and practical guidance

Proper theoretical descriptions of ground and excited states are critical for understanding molecular photophysics and photochemistry. Complex interactions in experimentally interesting molecular systems require multiple approximations of the underlying quantum mechanics to practically solve for various physical observables. While high-level calculations of small molecular systems provide very accurate excitation energies, this accuracy does not always extend to larger systems or other properties. Because of this, the “best” method to study new molecules is not always clear, leading many researchers to default to inexpensive and easy-to-use black-box methods. Unfortunately, even when these methods reproduce experimental excitation energies, it is not necessarily for the right reasons. Without accurate descriptions of the underlying physics, it becomes challenging to understand new classes of molecules. Consequently, predicted properties and their trends may not offer reliable mechanistic understanding. This review is targeted at beginners in computational chemistry who are interested in studying excited-state properties. A brief overview of common ground- and excited-state methods are covered for easy reference during the comparison of methods. The primary focus of this review is to compare the accuracy of these methods for several important classes of chromophores. The performance and accuracy of each method are explored to provide practitioners a road map on what methods work well for different molecular systems and identify further work that needs to be done in the field.