<div>It is well known that inclusion of dynamical correlation is needed in order to reach quantitative agreement with experiment for molecular systems with multi-reference character. In this work, we start from a non-orthogonal configuration interaction (NOCI) framework that accounts for the static correlation and incorporate dynamical correlation by including singles and doubles excitations out of each reference determinant resulting in a NOCISD wavefunction. The equations defining the NOCISD wavefunction commonly require the solution a poorly condition generalized eigenvalue problem, which we avoid by projecting the equations to a small dimension space defined by the CISD eigenvectors of each reference determinant. We show that NOCISD results are in good qualitative agreement with other state-of-the-art method for challenging problems such as the electron transfer in the ethylene dimer radical cation and LiF, as well as the description of the Jahn-Teller distortion in the cyclopentadienyl and nitrogen trioxide radicals.</div>

Quantum chemistry methods that describe excited states on the same footing as the ground state are generally scarce. In previous work, (Gill et al.

Quantum chemistry methods that describe excited states on the same footing as the ground state are generally scarce. In previous work, Gill et al. (J. Phys. Chem. A 112, 13164 (2008)) and later Sundstrom and Head-Gordon (J. Chem. Phys. 140, 114103 (2014)) considered excited states resulting from a non-orthogonal configuration interaction (NOCI) on stationary solutions of the Hartree–Fock equations. We build upon those contributions and present the state-averaged resonating Hartree–Fock (sa-ResHF) method, which differs from NOCI in that spin-projection and orbital relaxation effects are incorporated from the onset. Our results in a set of small molecules (alanine, formaldehyde, acetaldehyde, acetone, formamide, and ethylene) suggest that sa-ResHF excitation energies are a notable improvement over configuration interaction singles (CIS), at a mean-field computational cost. The orbital relaxation in sa-ResHF, in the presence of a spin-projection operator, generally results in excitation energies that are closer to the experimental values than the corresponding NOCI ones.

<div>It is well known that inclusion of dynamical correlation is needed in order to reach quantitative agreement with experiment for molecular systems with multi-reference character. In this work, we start from a non-orthogonal configuration interaction (NOCI) framework that accounts for the static correlation and incorporate dynamical correlation by including singles and doubles excitations out of each reference determinant resulting in a NOCISD wavefunction. The equations defining the NOCISD wavefunction commonly require the solution a poorly condition generalized eigenvalue problem, which we avoid by projecting the equations to a small dimension space defined by the CISD eigenvectors of each reference determinant. We show that NOCISD results are in good qualitative agreement with other state-of-the-art method for challenging problems such as the electron transfer in the ethylene dimer radical cation and LiF, as well as the description of the Jahn-Teller distortion in the cyclopentadienyl and nitrogen trioxide radicals.</div>

The de novo computational design of proteins with predefined three-dimensional structure is becoming much more routine due to advancements both in force fields and algorithms. However, creating designs with functions beyond folding is more challenging. In that regard, the recent design of small beta barrel proteins that activate the fluorescence of an exogenous small molecule chromophore (DFHBI) is noteworthy. These proteins, termed mini Fluorescence Activating Proteins (mFAPs), have been shown increase the brightness of the chromophore more than 100-fold upon binding to the designed ligand pocket. The design process created a large library of variants with different brightness levels but gave no rational explanation for why one variant was brighter than another. Here we use quantum mechanics and molecular dynamics simulations to investigate how molecular flexibility in the ground and excited states influences brightness. We show that the ability of the protein to resist dihedral angle rotation of the chromophore is critical for predicting brightness. Our simulations suggest that the mFAP/DFHBI complex has a rough energy landscape, requiring extensive ground-state sampling to achieve converged predictions of excited-state kinetics. While computationally demanding, this roughness suggests that mFAP protein function can be enhanced by reshaping the energy landscape towards states that better resist DFHBI bond rotation.