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Wallingford Center, CT, United States

This work reports the first implementation of the frequency dependent linear response (LR) function for the coupled cluster singles and doubles method (CCSD) combined with the polarizable continuum model of solvation for the calculation of frequency dependent properties in solution. In particular, values of static and dynamic polarizability as well as specific rotation are presented for various test molecules. Model calculations of polarizability show that a common approximation used in the definition of the LR function with solvation models recovers over 70% of the full response while maintaining a computational cost comparable to gas phase LR-CCSD. Calculations of specific rotation for three compounds for which gas phase methods predict the wrong sign of the rotation show that accounting for the electronic response of the solvent may be essential to assign the correct absolute configuration of chiral molecules. © 2013 AIP Publishing LLC. Source

Caricato M.,Gaussian Inc
Journal of Chemical Theory and Computation | Year: 2012

The effect of the solvent on the structure of a molecule in an electronic excited state cannot be neglected. However, the computational cost of including explicit solvent molecules around the solute becomes rather onerous when an accurate method such as the equation of motion coupled cluster singles and doubles (EOM-CCSD) is employed. Solvation continuum models like the polarizable continuum model (PCM) provide an efficient alternative to explicit models, since the solvent conformational average is implicit and the solute-solvent mutual polarization is naturally accounted for. In this work, the coupling of EOM-CCSD and PCM in a state specific approach is presented for the evaluation of energy and analytic energy gradients. Also, various approximations are explored to maintain the computational cost comparable to gas phase EOM-CCSD. Numerical examples are used to test the different schemes. © 2012 American Chemical Society. Source

The calculation of vertical electronic transition energies of molecular systems in solution with accurate quantum mechanical methods requires the use of approximate and yet reliable models to describe the effect of the solvent on the electronic structure of the solute. The polarizable continuum model (PCM) of solvation represents a computationally efficient way to describe this effect, especially when combined with coupled cluster (CC) methods. Two formalisms are available to compute transition energies within the PCM framework: State-Specific (SS) and Linear-Response (LR). The former provides a more complete account of the solute-solvent polarization in the excited states, while the latter is computationally very efficient (i.e., comparable to gas phase) and transition properties are well defined. In this work, I review the theory for the two formalisms within CC theory with a focus on their computational requirements, and present the first implementation of the LR-PCM formalism with the coupled cluster singles and doubles method (CCSD). Transition energies computed with LR- and SS-CCSD-PCM are presented, as well as a comparison between solvation models in the LR approach. The numerical results show that the two formalisms provide different absolute values of transition energy, but similar relative solvatochromic shifts (from nonpolar to polar solvents). The LR formalism may then be used to explore the solvent effect on multiple states and evaluate transition probabilities, while the SS formalism may be used to refine the description of specific states and for the exploration of excited state potential energy surfaces of solvated systems. © 2013 AIP Publishing LLC. Source

Hratchian H.P.,Gaussian Inc | Kraka E.,Southern Methodist University
Journal of Chemical Theory and Computation | Year: 2013

The reaction path connects a chemical potential energy landscape and the conceptual descriptions of chemical mechanisms and reactivity. In recent years, a class of predictor-corrector integrators has been developed and shown to provide an excellent compromise between computational efficiency and numerical accuracy. Models based on projected frequencies along the reaction path and coupling matrix elements, such as Reaction Path Hamiltonian (RPH) and Unified Reaction Valley Approach (URVA), require highly accurate integration of the reaction path. In this report, the Euler Predictor-Corrector (EulerPC) and Hessian-based Predictor-Corrector (HPC) methods are shown to be inadequate for studying reaction path curvature, which is a central component of the RPH and URVA models. The source of this apparent failure is explored, and a solution is developed. Importantly, the resulting enhanced EulerPC and HPC integrators do not require more intensive CPU or memory requirements than their predecessors. © 2013 American Chemical Society. Source

Hratchian H.P.,Gaussian Inc
Journal of Chemical Theory and Computation | Year: 2012

Projected frequencies along a reaction pathway are necessary for computing reaction rates using variational transition state theory or reaction path Hamiltonian methods. The projected frequency analysis is quite sensitive to the accuracy of the reaction path integration. This work demonstrates that second- and first-order predictor-corrector reaction path integrators can be used for computing projected frequencies with high confidence. It is shown that these methods perform equally well with a variety of numerical integration step sizes and that, without a substantive loss in accuracy, Hessian updating can be used with both methods for stepping along the reaction path at points between those where projected frequencies are required. © 2012 American Chemical Society. Source

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