64 Search Results

Achieving chemical accuracy for molecular simulations remains a central challenge in computational chemistry. Here, we present an embedded correlated wavefunction transfer learning (ECW-TL) framework for accurately simulating molecular dynamics in the condensed phase. ECW-TL incorporates high-level electron exchange and correlation effects in ECW theory while preserving the training and computational efficiency of machine-learned interatomic potentials. We demonstrate the framework on Ca²⁺–CO₃^2– ion pairing in aqueous solution, a key process underlying CO₂ mineralization in seawater. As proof of principle, we first show that fine-tuning a DFT-revPBE-D3(BJ) baseline model with embedded-DFT-SCAN data reproduces the DFT-SCAN free-energy surface within 1 kcal/mol across allmore » solvation states. Extending the framework to embedded MP2 and localized natural-orbital CCSD(T) further refines the free-energy profile, revealing the crucial role of exact electron exchange and correlation in determining ion-pair stability and structure. The computed ion-pair association free energy is in quantitative agreement with experimental measurements, further validating the accuracy of the ECW-TL framework. ECW-TL thus provides a general, data-efficient route for transferring CW accuracy to efficient simulations of complex aqueous and interfacial chemical processes.« less

Carbon–carbon (C–C) bond formation is necessary for hydrocarbon (and oxygenate) synthesis beyond methane (and formate/formic acid) during electrochemical CO and CO₂ reduction (ECOR and ECO₂R). Cu has notable ability to form hydrocarbons compared to other pure metals. In particular, the (100) facet of face-centered cubic Cu forms ethylene competitively with H₂ and methane during both ECOR and ECO₂R. Past simulations based on density functional theory (DFT) with standard exchange-correlation functional approximations predict fast nonelectroactive C–C bond formation channels involving adsorbed (*) CO together with another *CO, formyl (*CHO), or hydroxymethylidyne (*COH), forming OC*–*CO, OC*–CHO*, and OC*–*COH, respectively. Such simulations supportmore » the prevailing hypothesis that emergence of C₂ products is kinetically determined at the early stages of the reduction chemistry. Here we show, via simulations with more accurate many-body, i.e., “correlated”, wavefunction theory (enabled by an embedding scheme), that the coupling of *CO with a *CO or a *COH (previously predicted at the same level of theory to kinetically dominate over *CHO as the one-electron reduction product of *CO) is highly activated (kinetically impeded), with free energy barriers >1 eV, in contradiction to previous DFT-based simulations. Intriguingly, we find that the coupling of two adjacent *COHs incurs only a small barrier (<0.3 eV) and is exoergic (< –1 eV); however, given the predicted low surface mobility of *COH, the emergence of HOC*–*COH is also improbable, at least at low *COH coverages. We therefore conclude that it is highly unlikely for *CO to participate in nonelectroactive C–C bond formation on pristine Cu(100), contrary to conventional wisdom, and that the energetically favorable *COH dimerization may occur only after substantial buildup of *COH on the surface.« less

Formation of hydrocarbons containing two or more carbon atoms (C₂₊) during heterogeneous electrochemical CO and CO₂ reduction (ECOR and ECO₂R) only occurs, among pure metals, on Cu electrodes. Moreover, the activity and selectivity is facet dependent, with Cu(100) generally preferentially forming ethylene over methane. Previously, we found via quantum-mechanics-based modeling that, unlike standard density functional theory, more accurate correlated wavefunction methods predict that non-electroactive coupling pathways involving two adsorbed COs (*CO) or a *CO and a *COH to form C–C bonds on Cu(100) are kinetically inhibited, with the former also thermodynamically unfavorable. Here, we extend that embedded complete active spacemore » second order perturbation theory (ECASPT2) study, further showing that electrochemical coupling of two *COs to form an anionic dimer [OC*–*CO]^(1+δ)–, followed by protonation to form [OC*–*COH]^δ−, is not kinetically competitive with the reduction of *CO to *COH at relevant ECO/CO₂R potentials. Our simulations therefore suggest that the ability of Cu(100) to electrochemically synthesize C₂₊ molecules from CO and CO₂ is unlikely to be via *CO, at least on pristine Cu(100). Instead, hydrogenated CO species (*COH, *CH_xOH, or *CH_x) are most likely to be the key intermediates in C–C bond formation.« less

Density functional embedding theory (DFET) enables use of electronic structure methods with higher accuracy than density functional theory in a local region, with applications thus far ranging from (photo/electro)catalysis to reactions in solution. DFET partitions a large collection of atoms into smaller groups that interact via a shared embedding (interaction) potential V_emb, determined via functional optimization. The optimized effective potential (OEP) process used to optimize V_emb is time-consuming and becomes a computational bottleneck due to sharp, oscillating features of V_emb near nuclei. Here, similar to pseudopotential theory, by reconstructing electron densities used in the OEP process from smoother pseudo-valence-only (PVO)more » electron densities as proxies for total densities of the full system and subsystems, we can retain accuracy in the embedded electronic structure calculations while potentially reducing the overhead of V_emb construction, within the projector augmented-wave (PAW) formalism. We explore three different chemical reactions as exemplars to test PVO–DFET, namely, H₂ dissociative adsorption on a Cu(111) surface, H₂O adsorption on a Pt(111) surface, and aqueous [Ca²⁺–SO₄^2–] ion-pair formation. The PVO approximation works well for all three systems with minimal loss of accuracy (∼10–70 meV error relative to the original exact-derivative (ED) approach) while accelerating V_emb generation for the Cu and Pt systems respectively by 20× and 5×. Given proper numerical convergence parameters, the spatial distributions of differences between PVO- and ED-based V_emb outside the core regions are small, explaining the exceptional agreement between the two approaches. Finally, we anticipate that this more efficient PVO–DFET approximation will be useful whenever computation of V_emb is much more expensive than subsequent embedded high-level electron correlation calculations.« less

The random phase approximation (RPA), a method for treating electron correlation, has been shown to be superior to standard density functional theory (DFT) approximations in numerous cases. However, the RPA’s computational cost is substantially higher than that of DFT, particularly restricting its application to extended surfaces. The recently introduced embedded RPA (emb-RPA) approach [Wei et al., J. Chem. Phys. 159(19), 194108 (2023)] reduces this computational cost by approximately two orders of magnitude. While previous applications of emb-RPA focused on non-spin-polarized systems, here we extend the approach to ferromagnetic ones. Unlike other embedded correlated wavefunction methods, such as embedded complete activemore » space self-consistent field theory, emb-RPA is advantageous for spin-polarized systems because the RPA is compatible with unrestricted DFT solutions, which are eigenfunctions of the spin angular momentum operator S_z but not the total spin-squared operator S². By applying emb-RPA with specific magnetization constraints, we achieved a speedup of two to three orders of magnitude (one order when accounting for the one-time embedding potential optimization cost) with only small errors (∼50 meV) compared to full periodic RPA. Moreover, emb-RPA significantly reduces the over-binding errors of DFT approximations. In conclusion, we anticipate that the acceleration enabled by the spin-polarized emb-RPA approach will broaden the applicability of RPA to magnetic materials.« less

Ammonia (NH₃) is an energy-rich molecule that is routinely synthesized from nitrogen (N₂) and hydrogen (H₂). NH₃’s more favorable physical properties compared to H₂ suggests it may offer a way to more conveniently store, transport, and, when needed, extract H₂ via thermal decomposition. However, the high kinetic barrier and endoergicity to decompose to H₂ and N₂ require high temperatures. The standard reaction free energy indicates nearly 100% thermodynamic conversion to the diatomic molecules only at ~673 K and higher. However, even at these temperatures, a catalyst, e.g., iron (Fe), is needed for favorable kinetic conversion. Here, in this study, wemore » explore via density functional theory the kinetics of NH₃ decomposition on the most stable facet of body-centered cubic Fe, namely, (110), under typical high-temperature and finite-pressure operando conditions. We predict coverage-dependent energetics of elementary surface reactions, often neglected in atomic-scale modeling. From these models, we find the recombinative desorption of adsorbed N as N₂ is rate-determining at 573.15–773.15 K and even at an extreme case of 1173.15 K. From microkinetic modeling, we find that the steady-state turnover frequencies (TOFs) for N₂ and H₂ generation rates (r$$_{H_2}$$) depend exponentially on temperature. The catalyst achieves a steady-state TOF of 36.4 s^–1 and an r$$_{H_2}$$ of 0.107 μmol cm^–2 s^–1 for a feed of 1.8 bar NH₃ with 0.2 bar H₂ at 1173.15 K. However, at 773.15 K, with the same feed composition and velocity, the steady-state TOF and r$$_{H_2}$$ decrease to 0.14 s^–1 and 4.10 × 10^–4 μmol cm^–2 s^–1, respectively, as the process is significantly hindered by slow N₂ desorption. Although at first glance counterintuitive, our simulations suggest that surface modifications that reduce Fe’s reactivity toward NH_x species should enhance its overall NH₃ decomposition activity.« less

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to investigate methanol (CH₃OH) adsorption and reaction on the GaP(110) surface. Exposure of CH₃OH to GaP(110) at room temperature led to the formation of at least four different surface species as indicated by analysis of C 1s and O 1s XPS features. By combining AP-XPS data with density functional theory calculations, the surface species were identified as methoxy (CH₃O*), formaldehyde (CH₂O*), and paired methanol (p-CH₃O*H) and methoxy (p-CH₃O*) species, where “paired” means that they belong to a hydrogen-bonded methoxy-methanol complex. Asterisk * here indicates an adsite. The formation of CH₂O* via themore » dehydrogenation of CH₃O* was shown to be limited by the availability of vacant phosphorus (P) sites on GaP(110). With an increase in CH₃OH pressure, the fractional coverage of CH₃O* species reached 0.55, and the surface P sites were completely saturated with hydrogen. Under a constant CH₃OH pressure of 0.5 Torr, the surface concentration of the paired species and of CH₂O* remained constant until 400 K. At higher temperatures, thermally driven reactions led to a significant increase in the concentration of surface CH_x* species, which suggests that C-O bond cleavage of the CH₃O group is the dominant decomposition mechanism on GaP(110). In conclusion, based on the reactivity of GaP(110) toward CH₃OH dehydrogenation, elevated temperatures and CH₃OH pressures may be used to functionalize this surface.« less

Ammonia (NH₃) is one of the most important industrial chemicals. The conventional NH₃ synthesis method-the Haber–Bosch process-converts atmospheric nitrogen (N₂) into NH₃ using H₂ with an iron (Fe) catalyst. However, this process requires high pressures (100–200 atm) and temperatures (700–800 K) near thermal equilibrium. Recently, Fe-based nanocatalysts have been reported to produce promising NH₃ yields under atmospheric pressures and temperature-modulated nonequilibrium conditions. Understanding the mechanism of nonequilibrium catalysis with programmed temperature variation could help to optimize this fully electrified and less energy-intensive process. Although reactive molecular dynamics (RMD) simulations can be a useful tool to model nonequilibrium catalytic processes, theymore » require the development of accurate force fields (i.e., interatomic potentials). Here, we present a machine-learned (ML) force field within the Deep Potential MD (DPMD) framework, trained using periodic density functional theory (DFT) calculations, to model NH₃ synthesis on Fe catalysts with various surface adsorbates such as *N, *H, *N₂, *H₂, *NH, *NH₂, and *NH₃. Here, we generated the DFT data from static models of elementary reactions on the most stable (110) surface of body-centered cubic Fe, which then were augmented by data from constant number of particles–volume–temperature (NVT) DFT-MD trajectories at various temperatures. Finally, we utilized the fully optimized ML force field to investigate reaction dynamics at an Fe(110) surface at linearly increasing temperatures using NVT-DPMD simulations. Our simulations indicate that pulsed temperature ramping could prove favorable for NH3 synthesis. For example, we conducted ramping under multiple sets of conditions: (i) from 900 to 1200 K over periods of 0.1–0.3 ns for Fe surfaces precovered with N or NH along with H; and (ii) from 300 to 600 K over 0.1–0.3 ns for Fe surfaces precovered with NH₃. While our simulations so far are limited to short time scales (very rapid heating), these observations shed light on the mechanism of the high NH₃ synthesis rate achieved in a novel temperature-modulated nonequilibrium catalytic reactor using pulsed heating and cooling.« less

Here, solutions and minerals containing sulfate (SO₄^2-), and Ca²⁺ and Mg²⁺ cations, are ubiquitous throughout the lithosphere and are significant components of seawater, thus presenting a prototypical system for the study of strong electrolytes and crystal nucleation mechanisms. However, despite their relative abundance, key questions remain unanswered about the most fundamental atomic-level steps of their mineralization pathways and aqueous dynamics. Here, we carry out enhanced sampling multi-level molecular dynamics (MD) embedded correlated wavefunction theory simulations to elucidate ion-pairing mechanisms for Mg–SO₄ and Ca–SO₄ in concentrated aqueous solution, accurately capturing effects arising from both structural dynamics and electron exchange–correlation. We predictmore » contact-ion-pair formation to be barrierless and highly exoergic for Ca–SO₄, in agreement with its minimal solubility, whereas for Mg–SO₄, solvent-shared and contact ion pairs have similar free energies, qualitatively consistent with its higher solubility. Finally, we demonstrate that brief high-temperature pre-equilibration may be utilized to accelerate convergence of free energies in blue-moon-ensemble enhanced-sampling MD.« less

Here, this essay aims to suggest research areas to which chemists and others in related fields can contribute, to help preserve and sustain the world for future generations by addressing problems of global importance.