MSINDO-sCIS: A New Method for the Calculation of Excited States of Large Molecules

Benchmarks for Electronically Excited States with CASSCF Methods

The accuracy of three different complete active space (CAS) self-consistent field (CASSCF) methods is investigated for the electronically excited-state benchmark set of Schreiber , M. ; et al. J. Chem. Phys. 2008 , 128 , 134110 . Comparison of the CASSCF linear response (LR) methods MC-RPA and MC-TDA and the state-averaged (SA) CASSCF method is made for 122 singlet excitation energies and 69 oscillator strengths. Of all CASSCF methods, when considering the complete test set, MC-RPA performs best for both excitation energies and oscillator strengths with a mean absolute error (MAE) of 0.74 eV and 51%, respectively. MC-TDA and SA-CASSCF show a similar accuracy for the excitation energies with a MAE of ∼1 eV with respect to more accurate coupled cluster (CC3) excitation energies. The opposite trend is observed for the subset of n → π* excitation energies for which SA-CASSCF exhibits the least deviations (MAE 0.65 eV). By looking at s-tetrazine in more detail, we conclude that better performance for the n → π* SA-CASSCF excitation energies can be attributed to a fortunate error compensation. For oscillator strengths, SA-CASSCF performs worst for the complete test set (MAE 100%) as well as for the subsets of n → π* (MAE 192%) and π → π* excitations (MAE 84.9%). In general, CASSCF gives the worst performance for excitation energies of all excited-state ab initio methods considered so far due to lacking the major part of dynamic electron correlation, though MC-RPA and TD-DFT (BP86) show similar performance. Among all LR-type methods, LR-CASSCF oscillator strengths are the ones with the least accuracy for the same reason. As state-specific orbital relaxation effects are accounted for in LR-CASSCF, oscillator strengths are significantly more accurate than those of MS-CASPT2. Our findings should encourage further developments of response theory-based multireference methods with higher accuracy and feasibility.

Conductor-like screening model for relaxed excited states: Implementation in the semiempirical method MSINDO

Two approaches to treat solvent polarization and reorientation effects for excited states of molecules and surfaces have been implemented in the recently developed MSINDO-sCIS method (Gadaczek, Krause, Hintze, Bredow, J. Chem. Theory Comput. 2011, 7, 3675). They allow for an efficient calculation of analytical energy gradients and hence open the opportunity to investigate fluorescence effects or photochemical reactions in solution for large molecules that are difficult to treat with high-level methods. Both approaches are based on the conductor-like screening model (COSMO) (Klamt and Schüürmann, J. Chem. Soc., Perkin Trans. 1993, 2, 799) in combination with the configuration interaction singles (CIS) method (Foresman, Head-Gordon, Pople, and Frisch, J. Phys. Chem. 1992, 96, 135). The paper gives a brief outline of the theoretical background. As a first application, solvent shifts of three well-studied, environment-sensitive fluorescent dyes (Kucherak, Didier, Mély, and Klymchenko, J. Phys. Chem. Lett. 2010, 1, 616) have been calculated and compared with experimental results and standard time-dependent density functional theory. A statistical evaluation of MSINDO-COSMO-sCIS is provided for a set of 39 molecules suggested recently by Jacquemin et al. (Jacquemin, Planchat, Adamo, and Mennucci, J. Chem. Theory Comput. 2012, 8, 2359). Calculated vertical and adiabatic excitation energies and fluorescence energies are compared to experimental data.

Full-Quantum chemical calculation of the absorption maximum of bacteriorhodopsin: a comprehensive analysis of the amino acid residues contributing to the opsin shift

Herein, the absorption maximum of bacteriorhodopsin (bR) is calculated using our recently developed method in which the whole protein can be treated quantum mechanically at the level of INDO/S-CIS//ONIOM (B3LYP/6-31G(d,p): AMBER). The full quantum mechanical calculation is shown to reproduce the so-called opsin shift of bR with an error of less than 0.04 eV. We also apply the same calculation for 226 different bR mutants, each of which was constructed by replacing any one of the amino acid residues of the wild-type bR with Gly. This substitution makes it possible to elucidate the extent to which each amino acid contributes to the opsin shift and to estimate the inter-residue synergistic effect. It was found that one of the most important contributions to the opsin shift is the electron transfer from Tyr185 to the chromophore upon excitation. We also indicate that some aromatic (Trp86, Trp182) and polar (Ser141, Thr142) residues, located in the vicinity of the retinal polyene chain and the β-ionone ring, respectively, play an important role in compensating for the large blue-shift induced by both the counterion residues (Asp85, Asp212) and an internal water molecule (W402) located near the Schiff base linkage. In particular, the effect of Trp86 is comparable to that of Tyr185. In addition, Ser141 and Thr142 were found to contribute to an increase in the dipole moment of bR in the excited state. Finally, we provide a complete energy diagram for the opsin shift together with the contribution of the chromophore-protein steric interaction.