Adrian W. Lange

Both intra- and interstrand charge-transfer excited states in aqueous B-DNA are present at energies comparable to, or just above, the (1)pipi* excitonic bright states.

Vertical electronic excitations in model systems representing single- and double-stranded B-DNA are characterized using electronic structure theory, including both time-dependent density functional theory (TD-DFT) and correlated wave function techniques. Previous TD-DFT predictions of charge-transfer (CT) states well below the optically bright (1)pipi* states are shown to be artifacts of the improper long-range behavior of standard density-functional exchange approximations, which we rectify here using a long-range correction (LRC) procedure. For nucleobase dimers (hydrogen-bonded or pi-stacked), TD-LRC-DFT affords vertical excitation energies in reasonable agreement with the wave function methods, not only for the (1)npi* and (1)pipi* states but also for the CT states, and qualitatively reproduces well-known base-stacking effects on the absorption spectrum of DNA. The emergence of (1)pipi* Frenkel exciton states, localized on a single strand, is clearly evident, and these states (rather than low-energy CT states) are primarily responsible for the fact that DNAs absorption spectrum exhibits a red tail that is absent in monomer absorption spectra. For B-DNA in aqueous solution, the low-energy tail of the CT band (representing both intra- and interstrand CT states) appears at energies comparable to those of the optically bright (1)pipi* exciton states. In systems with more than one base pair, we also observe the emergence of delocalized, interstrand CT excitations, whose excitation energies may be significantly lower than the lowest CT excitation in a single base pair. Together, these observations suggest that a single Watson-Crick base pair is an inadequate model of the photophysics of B-DNA.

Charge-Transfer Excited States in a π-Stacked Adenine Dimer, As Predicted Using Long-Range-Corrected Time-Dependent Density Functional Theory

The lowest few electronic excitations of a pi-stacked adenine dimer in its B-DNA geometry are investigated, in the gas phase and in a water cluster, using a long-range-corrected version of time-dependent density functional theory (TD-DFT) that asymptotically incorporates Hartree-Fock exchange. Long-range correction is shown to eliminate the catastrophic underestimation of charge-transfer (CT) excitation energies that plagues conventional TD-DFT, at the expense of introducing one adjustable parameter, mu, that determines the length scale on which Hartree-Fock exchange is turned on. This parameter allows us to interpolate smoothly between hybrid density functionals and time-dependent Hartree-Fock theory. Excitation energies for CT states (in which an electron is transferred from one adenine molecule to the other) are found to increase dramatically as a function of mu. Uncorrected hybrid functionals underestimate the CT excitation energies, placing them well below the valence excitations, while time-dependent Hartree-Fock calculations place these states well above the valence states. Values for mu determined from certain benchmark calculations place the CT states well above the valence pipi* and npi* states at the Franck-Condon point.

A smooth, nonsingular, and faithful discretization scheme for polarizable continuum models: the switching/Gaussian approach.

Polarizable continuum models (PCMs) are a widely used family of implicit solvent models based on reaction-field theory and boundary-element discretization of the solute/continuum interface. An often overlooked aspect of these theories is that discretization of the interface typically does not afford a continuous potential energy surface for the solute. In addition, we show that discretization can lead to numerical singularities and violations of exact variational conditions. To fix these problems, we introduce the switching/Gaussian (SWIG) method, a discretization scheme that overcomes several longstanding problems with PCMs. Our approach generalizes a procedure introduced by York and Karplus [J. Phys. Chem. A 103, 11060 (1999)], extending it beyond the conductor-like screening model. Comparison to other purportedly smooth PCM implementations reveals certain artifacts in these alternative approaches, which are avoided using the SWIG methodology. The versatility of our approach is demonstrated via geometry optimizations, vibrational frequency calculations, and molecular dynamics simulations, for solutes described using quantum mechanics and molecular mechanics.

Simple Methods To Reduce Charge-Transfer Contamination in Time-Dependent Density-Functional Calculations of Clusters and Liquids

Using as benchmarks a series of increasingly large hydrated uracil clusters, we examine the nature and extent of charge-transfer (CT) contamination in condensed-phase, time-dependent density-functional theory. These calculations are plagued by a large number of spurious CT excitations at energies comparable to (and sometimes below) the valence excitation energies, even when hybrid density functionals are used. Spurious states below the first nπ* and ππ* states of uracil are observed in clusters as small as uracil-(H2O)4. Reasonable electronic absorption spectra can still be obtained, upon configurational averaging, despite pervasive CT contamination, but the spurious states add significantly to the cost of the calculations and severely complicate attempts to locate optically dark nπ* states. The extent of CT contamination is reduced substantially by introducing an electrostatic (point charge) description of an extended solvent network, even in cases where the region of solvent described by density functional theory is large (>120 atoms). Alternatively, CT contamination may be reduced by eliminating certain excitation amplitudes from the linear response equations, with minimal loss of accuracy (<0.1 eV) in the valence excitation energies.

Multi-state Approach to Chemical Reactivity in Fragment Based Quantum Chemistry Calculations

We introduce a multistate framework for Fragment Molecular Orbital (FMO) quantum mechanical calculations and implement it in the context of protonated water clusters. The purpose of the framework is to address issues of nonuniqueness and dynamic fragmentation in FMO as well as other related fragment methods. We demonstrate that our new approach, Fragment Molecular Orbital Multistate Reactive Molecular Dynamics (FMO-MS-RMD), can improve energetic accuracy and yield stable molecular dynamics for small protonated water clusters undergoing proton transfer reactions.

Improving Generalized Born Models by Exploiting Connections to Polarizable Continuum Models. II. Corrections for Salt Effects

A previous analytical investigation of the generalized Born (GB) implicit solvation model is extended to solvents of nonzero ionic strength. The GB model with salt effects (GB-SE) is shown to resemble the Debye-Hückel-like screening model (DESMO), a polarizable continuum model (PCM) that we have recently developed for salty solutions. DESMO may be regarded either as a generalization of the conductor-like PCM (C-PCM) that extends C-PCM to electrolyte solutions or alternatively as a generalization of Debye-Hückel theory to arbitrary cavity shapes. The connection between GB-SE and DESMO suggests how the former can be modified to account for the exclusion of mobile ions from the cavity interior, an effect that is typically absent in GB-SE models. We propose two simple GB-SE models that are exact for a point charge in a spherical cavity and that introduce the ability to account, albeit approximately, for the finite size of the mobile ions. The accuracy of these new models is demonstrated by applications to both model systems and real proteins. These tests also demonstrate the accuracy of the DESMO approach, as compared to more sophisticated PCMs developed for electrolyte solutions.

Response to “Comment on ‘A smooth, nonsingular, and faithful discretization scheme for polarizable continuum models: The switching/Gaussian approach’” [J. Chem. Phys.134, 117101 (2011)]

In response to the Comment by Scalmani and Frisch, we clarify certain claims made in the context of our “switching/Gaussian” discretization procedure. Furthermore, an explanation is proposed to explain observed similarities between this technique and the “continuous surface charge” method introduced by Scalmani and Frisch.