title: Polarizable QM/MM Multiconfiguration Self-Consistent Field Approach with State-Specific Corrections: Environment Effects on Cytosine Absorption Spectrum.
abstract: We present the formulation and implementation of a polarizable quantum mechanics/molecular mechanics (QM/MM) strategy to describe environment effects in multiconfiguration self-consistent field calculations. The strategy is applied to the calculation of the vertical absorption spectrum of cytosine in water. In our approach, mutual polarization of the solute and the solvent is solved self-consistently at the complete-active-space self-consistent-field (CASSCF) level, and the resulting set of charges and dipoles is used to calculate vertical excitation energies using the complete-active-space second-order perturbative (CASPT2) approach and its multistate (MS-CASPT2) variant. In order to treat multiple excited states, we converge the solvent polarization with respect to the state-averaged density of the solute. In order to obtain the final energies, however, we introduce a state-specific correction, where the solvent polarization is recomputed with the density of each state, and demonstrate that this correction brings the excitation energies closer to the values obtained with state-optimized orbitals. Comparison with PCM and nonpolarizable QM/MM calculations shows the importance of specific solute-solvent interactions and environment polarization in describing experiments. Overall, the calculated excitations for the π → π* states in water show good agreement with the experimental spectrum, whereas the n → π* appear at energies above 6 eV, approximately 1 eV higher than in the gas phase. Beyond solvents, the new method will allow studying the impact of heterogeneous biological environments in multiple excited states, as well as the treatment of multichromophoric systems where charge transfer and exciton states play important roles.
authors.affiliation: School of Chemistry, Beijing Institute of Technology , Beijing 100081, China.
authors.forename: Quansong
authors.initials: Q
authors.lastname: Li
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, University of Pisa , Via G. Moruzzi, 56124 Pisa, Italy.
authors.forename: Benedetta
authors.initials: B
authors.lastname: Mennucci
authors.affiliation: Department of Chemistry, Imperial College London , London SW7 2AZ, United Kingdom.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Robb
authors.affiliation: Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona , Campus Montilivi, 17071 Girona, Spain.
authors.forename: Lluís
authors.initials: L
authors.lastname: Blancafort
authors.affiliation: Departament de Fisicoquímica, Facultat de Farmàcia, Universitat de Barcelona , Av. Joan XXIII s/n, 08028 Barcelona, Spain.
authors.forename: Carles
authors.initials: C
authors.lastname: Curutchet
mesh_terms: D003596:Cytosine / Q000737:chemistry*; D004583:Electrons; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D003596:Cytosine