Effective Fragment Potentials for Flexible Molecules: Transferability of Parameters and Amino Acid Database

An accurate but efficient description of noncovalent interactions is a key to predictive modeling of biological and materials systems. The effective fragment potential (EFP) is an ab initio-based force field that provides a physically meaningful decomposition of noncovalent interactions of a molecular system into Coulomb, polarization, dispersion, and exchange-repulsion components. An EFP simulation protocol consists of two steps, preparing parameters for molecular fragments by a series of ab initio calculations on each individual fragment, and calculation of interaction energy and properties of a total molecular system based on the prepared parameters. As the fragment parameters (distributed multipoles, polarizabilities, localized wave function, etc.) depend on a fragment geometry, straightforward application of the EFP method requires recomputing parameters of each fragment if its geometry changes, for example, during thermal fluctuations of a molecular system. Thus, recomputing fragment parameters can easily become both computational and human bottlenecks and lead to a loss of efficiency of a simulation protocol. An alternative approach, in which fragment parameters are adjusted to different fragment geometries, referred to as “flexible EFP”, is explored here. The parameter adjustment is based on translations and rotations of local coordinate frames associated with fragment atoms. The protocol is validated on extensive benchmark of amino acid dimers extracted from molecular dynamics snapshots of a cryptochrome protein. A parameter database for standard amino acids is developed to automate flexible EFP simulations in proteins. To demonstrate applicability of flexible EFP in large-scale protein simulations, binding energies and vertical electron ionization and electron attachment energies of a lumiflavin chromophore of the cryptochrome protein are computed. The results obtained with flexible EFP are in a close agreement with the standard EFP procedure but provide a significant reduction in computational cost

The effect of pi-stacking, h-bonding, and electrostatic interactions on the ionization energies of nucleic acid bases: adenine-adenine, thymine-thymine and adenine-thymine dimers

A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Adiabatic and vertical ionization energies(IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions stronglyaffect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largestchanges in vertical IEs (0.4 eV) occur in asymmetric h-bonded and symmetric pi- stacked isomers, whereas in the lowest-energy symmetric h-bonded dimer the shiftin IEs is much smaller (0.1 eV). The origin of the shift and the character of the ionized states is different in asymmetric h-bonded and symmetric stacked isomers. Inthe former, the initial hole is localized on one of the fragments, and the shift is due to the electrostatic stabilization of the positive charge of the ionized fragment by thedipole moment of the neutral fragment. In the latter, the hole is delocalized, and the change in IE is proportional to the overlap of the fragments' MOs. The shifts in AAare much smaller due to a less effcient overlap and a smaller dipole moment. The ionization of the h-bonded dimers results in barrierless (or nearly barrierless) protontransfer, whereas the pi-stacked dimers relax to structures with the hole stabilized by the delocalization or electrostatic interactions

Electronic Structure and Spectroscopy of Nucleic Acid Bases: Ionization Energies, Ionization-Induced Structural Changes, and Photoelectron Spectra

We report high-level ab initio calculations and single-photon ionization mass spectrometry study of ionization of adenine (A), thymine (T), cytosine (C) and guanine (G). For thymine and adenine, only the lowest-energy tautomers were considered, whereas for cytosine and guanine we characterized five lowest-energy tautomeric forms. The first adiabatic and several vertical ionization energies were computed using equation-of-motion coupled-cluster method for ionization potentials with single and double substitutions. Equilibrium structures of the cationic ground states were characterized by DFT with the {omega}B97X-D functional. The ionization-induced geometry changes of the bases are consistent with the shapes of the corresponding molecular orbitals. For the lowest-energy tautomers, the magnitude of the structural relaxation decreases in the following series G > C > A > T, the respective relaxation energies being 0.41, 0.32, 0.25 and 0.20 eV. The computed adiabatic ionization energies (8.13, 8.89, 8.51-8.67 and 7.75-7.87 eV for A,T,C and G, respectively) agree well with the onsets of the photoionization efficiency (PIE) curves (8.20 {+-} 0.05, 8.95 {+-} 0.05, 8.60 {+-} 0.05 and 7.75 {+-} 0.05 eV). Vibrational progressions for the S{sub 0}-D{sub 0} vibronic bands computed within double-harmonic approximation with Duschinsky rotations are compared with previously reported experimental photoelectron spectra

The effect of microhydration on ionization energies of thymine

A combined theoretical and experimental study of the effect of microhydration on ionization energies (IEs) of thymine is presented. The experimental IEs are derived from photoionization efficiency curves recorded using tunable synchrotron VUV radiation. The onsets of the PIE curves are 8.85+-0.05, 8.60+-0.05, 8.55+-0.05, and 8.40+-0.05 eV for thymine, thymine mono-, di-, and tri-hydrates, respectively. The computed (EOM-IP-CCSD/cc-pVTZ) AIEs are 8.90, 8.51, 8.52, and 8.35 eV for thymine and the lowest isomers of thymine mono-, di-, and tri-hydrates. Due to large structural relaxation, the Franck-Condon factors for the 0<-- 0 transitions are very small shifting the apparent PIE onsets to higher energies. Microsolvation strongly affects IEs of thymine -- addition of each water molecule reduces the first vertical IE by 0.10-0.15 eV. The adiabatic IE decreases even more (up to 0.4 eV). The magnitude of the effect varies for different ionized states and for different isomers. For the ionized states that are localized on thymine the dominant contribution to the IE reduction is the electrostatic interaction between the delocalized positive charge on thymine and the dipole moment of the water molecule

Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

The effect of pi-stacking, h-bonding, and electrostatic interactions on the ionization energies of nucleic acid bases: adenine-adenine, thymine-thymine and adenine-thymine dimers

A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Adiabatic and vertical ionization energies (IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions strongly affect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largest changes in vertical IEs (0.4 eV) occur in asymmetric h-bonded and symmetric pi- stacked isomers, whereas in the lowest-energy symmetric h-bonded dimer the shift in IEs is much smaller (0.1 eV). The origin of the shift and the character of the ionized states is different in asymmetric h-bonded and symmetric stacked isomers. In the former, the initial hole is localized on one of the fragments, and the shift is due to the electrostatic stabilization of the positive charge of the ionized fragment by the dipole moment of the neutral fragment. In the latter, the hole is delocalized, and the change in IE is proportional to the overlap of the fragments' MOs. The shifts in AA are much smaller due to a less effcient overlap and a smaller dipole moment. The ionization of the h-bonded dimers results in barrierless (or nearly barrierless) proton transfer, whereas the pi-stacked dimers relax to structures with the hole stabilized by the delocalization or electrostatic interactions

Free Energies of Redox Half-Reactions from First-Principles Calculations

Quantitative prediction of the energetics of redox half-reactions is still a challenge for modern computational chemistry. Here, we propose a simple scheme for reliable calculations of vertical ionization and attachment energies, as well as of redox potentials of solvated molecules. The approach exploits linear response approximation in the context of explicit solvent simulations with spherical boundary conditions. It is shown that both vertical ionization energies and vertical electron affinities, and, consequently redox potentials, exhibit linear dependence on the inverse radius of the solvation sphere. The explanation of the linear dependence is provided, and an extrapolation scheme is suggested. The proposed approach accounts for the specific short-range interactions within hybrid DFT and effective fragment potential approach as well as for the asymptotic system-size effects. The computed vertical ionization energies and redox potentials are in excellent agreement with the experimental values

First-Principles Calculations of the Energy and Width of the ²A_u Shape Resonance in <i>p</i>‑Benzoquinone: A Gateway State for Electron Transfer

Quinones are versatile biological electron acceptors and mobile electron carriers in redox processes. We present the first ab initio calculations of the width of the ²A_u shape resonance in the <i>para</i>-benzoquinone anion, the simplest member of the quinone family. This resonance state located at 2.5 eV above the ground state of the anion is believed to be a gateway state for electron attachment in redox processes involving quinones. We employ the equation-of-motion coupled-cluster method for electron affinity augmented by a complex-absorbing potential (CAP-EOM-EA-CCSD) to calculate the resonance position and width. The calculated width, 0.013 eV, is in excellent agreement with the width of the resonant peak in the photodetachment spectrum, thus supporting the assignment of the band to resonance excitation to the autodetaching ²A_u state. The methodological aspects of CAP-EOM-EA-CCSD calculations of resonances positions and widths in medium-sized molecules, such as basis set and CAP box size effects, are also discussed