The simplest variational method for treating electronic excited states, configuration interaction with single excitations (CIS), has been interfaced with the effective fragment potential (EFP) method to provide an effective and computationally efficient approach for studying the qualitative effects of solvents on the electronic spectra of molecules. Three different approaches for interfacing a non-self-consistent field (SCF) excited-state quantum mechanics (QM) method and the EFP method are discussed. The most sophisticated and complex approach (termed fully self consistent) calculates the excited-state electron density with fully self-consistent accounting for the polarization (induction) energy of effective fragments. The simplest approach (method 1) includes a strategy that indirectly adds the EFP perturbation to the CIS wave function and energy via modified Hartree−Fock molecular orbitals, so that there is no direct EFP interaction with the excited-state density. An intermediate approach (method 2) accomplishes the latter in a noniterative perturbative manner. Theoretical descriptions of the three approaches are presented, and test results of solvent-induced shifts using methods 1 and 2 are compared with fully ab initio values. These comparisons illustrate that, at least for the test cases examined here, modification of the ground-state Hartree−Fock orbitals is the largest and most important factor in the calculated solvent-induced shifts. Method 1 is then employed to study the aqueous solvation of coumarin 151 and compared with experimental measurements. PO Box 2206, Germantown, Maryland 20875 ReceiVed: February 27, 2010 ReVised Manuscript ReceiVed: May 20, 2010 The simplest variational method for treating electronic excited states, configuration interaction with single excitations (CIS), has been interfaced with the effective fragment potential (EFP) method to provide an effective and computationally efficient approach for studying the qualitative effects of solvents on the electronic spectra of molecules. Three different approaches for interfacing a non-self-consistent field (SCF) excited-state quantum mechanics (QM) method and the EFP method are discussed. The most sophisticated and complex approach (termed fully self consistent) calculates the excited-state electron density with fully self-consistent accounting for the polarization (induction) energy of effective fragments. The simplest approach (method 1) includes a strategy that indirectly adds the EFP perturbation to the CIS wave function and energy via modified Hartree-Fock molecular orbitals, so that there is no direct EFP interaction with the excited-state density. An intermediate approach (method 2) accomplishes the latter in a noniterative perturbative manner. Theoretical descriptions of the three approaches are presented, and test results of solvent-induced shifts using methods 1 and 2 are compared with fully ab initio values. These comparisons illustrate that, at least for the test cases examined here, modification of the groun...

The processes of etching and diffusion of atomic oxygen on the reconstructed Si(100)-2 × 1 surface are investigated using an embedded cluster QM/MM (Quantum Mechanics/Molecular Mechanics) method, called SIMOMM (Surface Integrated Molecular Orbital Molecular Mechanics). Hopping of an oxygen atom along the silicon dimer rows on a Si15H16 cluster embedded in an Si136H92 MM cluster model is studied using the SIMOMM/UB3LYP (unrestricted density functional theory (UDFT) with the Becke threeparameter Lee−Yang−Parr (B3LYP) hybrid functional) approach, the Hay−Wadt effective core potential, and its associated double-ζ plus polarization basis set. The relative energies at stationary points on the diffusion potential energy surface were also obtained with three coupled-cluster (CC) methods, including the canonical CC approach with singles, doubles, and noniterative quasi-perturbative triples (CCSD(T)), the canonical left-eigenstate completely renormalized (CR) analogue of CCSD(T), termed CR-CC(2,3), and the linear scaling variant of CR-CC(2,3) employing the cluster-in-molecule (CIM) local correlation ansatz, abbreviated as CIM-CR-CC(2,3). The pathway and energetics for the diffusion of oxygen from one dimer to another are presented, with the activation energy estimated to be 71.9 and 74.4 kcal/mol at the canonical CR-CC(2,3)/6-31G(d) and extrapolated, CIM-based, canonical CR-CC(2,3)/6-311G(d) levels of theory, respectively. The canonical and CIM CR-CC(2,3)/6-31G(d) barrier heights (excluding zero point vibrational energy contributions) for the etching process are both 87.3 kcal/mol. The processes of etching and diffusion of atomic oxygen on the reconstructed Si(100)-2 × 1 surface are investigated using an embedded cluster QM/MM (Quantum Mechanics/Molecular Mechanics) method, called SIMOMM (Surface Integrated Molecular Orbital Molecular Mechanics). Hopping of an oxygen atom along the silicon dimer rows on a Si 15 H 16 cluster embedded in an Si 136 H 92 MM cluster model is studied using the SIMOMM/UB3LYP (unrestricted density functional theory (UDFT) with the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid functional) approach, the Hay-Wadt effective core potential, and its associated double-plus polarization basis set. The relative energies at stationary points on the diffusion potential energy surface were also obtained with three coupled-cluster (CC) methods, including the canonical CC approach with singles, doubles, and noniterative quasi-perturbative triples (CCSD(T)), the canonical lefteigenstate completely renormalized (CR) analogue of CCSD(T), termed CR-CC(2,3), and the linear scaling variant of CR-CC(2,3) employing the cluster-in-molecule (CIM) local correlation ansatz, abbreviated as CIM-CR-CC(2,3). The pathway and energetics for the diffusion of oxygen from one dimer to another are presented, with the activation energy estimated to be 71.9 and 74.4 kcal/mol at the canonical CR-CC(2,3)/6-31G(d) and extrapolated, CIM-based, canonical CR-CC(2,3)/6-311G(d) levels of theory, respectively. The canonical and C...