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.

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.

Improving Generalized Born Models by Exploiting Connections to Polarizable Continuum Models. I. An Improved Effective Coulomb Operator.

We investigate the generalized Born (GB) implicit solvation model in comparison with polarizable continuum models (PCMs). We show that the GB model is intimately connected to the conductor-like PCM (C-PCM), a method that is accurate for high-dielectric solvents but less so for weakly polar and nonpolar solvents. The formal connection between C-PCM and the GB model suggests that C-PCM calculations place a limit on the accuracy that one should expect from GB models but also demonstrates that comparison of GB and C-PCM calculations directly interrogates the accuracy of the effective Coulomb operator that is used in the pairwise GB energy expression.

A simple polarizable continuum solvation model for electrolyte solutions.

We propose a Debye-Hückel-like screening model (DESMO) that generalizes the familiar conductor-like screening model (COSMO) to solvents with non-zero ionic strength and furthermore provides a numerical generalization of the Debye-Hückel model that is applicable to non-spherical solute cavities. The numerical implementation of DESMO is based upon the switching/Gaussian (SWIG) method for smooth cavity discretization, which we have recently introduced in the context of polarizable continuum models (PCMs). This approach guarantees that the potential energy is a smooth function of the solute geometry and analytic gradients for DESMO are reported here.

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.

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.

Charge-transfer excited states in a pi-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.