Charged-cell periodic DFT simulations via an impurity model based on density embedding: Application to the ionization potential of liquid water

Calculations of charged systems in periodic boundary conditions (PBC) are problematic because there are spurious interactions between the charges in different periodic images that can affect the physical picture. In addition, the intuitive limit of Coulomb interactions decaying to zero as the interacting charges are placed at infinite separation no longer applies, and for example total energies become undefined. Leveraging subsystem density functional theory (also known as density embedding) we define an impurity model that embeds a finite neutral or charged subsystem within an extended (infinite) surrounding subsystem. The combination of the impurity model and a consistent choice of the Coulomb reference provides us with an algorithm for evaluating the ionization potential (IP) in extended systems. We demonstrate our approach in a pilot calculation of the IP of liquid water, based on a configuration from a prior ab initio molecular dynamics (AIMD) simulation of liquid water (Genova et al., J. Chem. Phys. 2016, 144, 234105). The calculations with the impurity model capture the broadening on the ionization energies introduced by the interactions between the water molecules. Furthermore, the calculated average IP value (10.5 eV) compare favorably to experiments (9.9-10.06 eV) and very recent simulations based on the GW approximation (10.55 eV), while at the same time outperforming density embedding calculations carried out with a naïve handling of the electrostatic interactions (about 7 eV).     

A quantum‐chemical DFT study of the mechanism and regioselectivity of the 1,3‐dipolar cycloaddition reaction of nitrile oxide with electron‐rich ethylenes

The 1,3‐dipolar cycloaddition (13DC) reactions of nitrile‐oxide NO 1 with two ethylenes, enamine 2a and enamine 2b , were computationally studied using B3LYP/6‐31G(d) DFT methods. The two possible ortho and meta regioselective channels were characterized and analyzed. The moderate polarity of these 13DC reactions is related to the high nucleophilic character of both ethylenes, and the moderate electrophilic nature of the NO 1 , that accounts for the relatively low calculated activation energies. Analysis of different forms of energies along the different reaction channels indicates that the present 13DC reactions are completely ortho regioselective, accordingly to the experimental outcomes. Electron localization function analysis indicates that these 13DC reactions proceed via a nonconcerted (two‐stage) one‐step mechanism.     

Selenyl analog of the (Z)-C,N-diphenylnitrone as the TAC in [3 + 2] cycloaddition with nitroethene: A DFT computational study

Abstract

The molecular mechanism of [3?+?2] cycloaddition (32CA) process between selenyl analog of the (Z)-C,N-diphenylnitrone and nitroethene was examined on the basis of the M06-2X/6-311?+?G(d)(PCM) calculations. It was found that kinetic factors favor the formation of 4-nitro substituted cycloadducts in the reaction course. Additionally, the DFT computational study highlights the evidently polar nature of the title reaction. The mechanistic study also confirms the possibility of the existence of a zwitterionic intermediate in the reaction environment.