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  1. Free, publicly-accessible full text available September 4, 2025
  2. The chemical bond is the cornerstone of chemistry, providing a conceptual framework to understand and predict the behavior of molecules in complex systems. However, the fundamental origin of chemical bonding remains controversial, and has been responsible for fierce debate over the past century. Here we present a unified theory of bonding, using a separation of electron delocalization effects from orbital relaxation to identify three mechanisms -- node-induced confinement (typically associated with Pauli repulsion, though more general), orbital contraction and polarization -- that each modulate kinetic energy during bond formation. Through analysis of a series of archetypal bonds, we show that an exquisite balance of energy-lowering delocalizing and localizing effects are dictated simply by atomic electron configurations, nodal structure and electronegativities. The utility of this unified bonding theory is demonstrated by its application to explain observed trends in bond strengths throughout the periodic table, including main group and transition metal elements. 
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    Free, publicly-accessible full text available April 10, 2025
  3. Free, publicly-accessible full text available March 6, 2025
  4. Energy decomposition analysis (EDA) is a useful method to unravel an intermolecular interaction energy into chemically meaningful components such as geometric distortion, frozen interactions, polarization, and charge transfer. A further decomposition of the polarization (POL) and charge transfer (CT) energy into fragment-wise contributions would be useful to understand the significance of each fragment during these two processes. To complement the existing exact pairwise decomposition of the CT term, this work describes formulation and implementation of a non-perturbative polarization analysis that decomposes the POL energy into an exactly fragment-wise additive sum based on the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). These fragment-wise contributions can be further decomposed into chemically intuitive molecular orbital pairs using complementary occupied-virtual pairs (COVP) analysis. A very useful phase convention is established for each COVP such that constructive interference of occupied and virtual corresponds to electron flow into that region, whilst destructive interference corresponds to electron outflow. A range of model problems are used to demonstrate that the polarization process is typically a collective behavior of the electrons that is quite different from the charge transfer process. This provides another reason in addition to their different distance-dependence on fragment separation for separating these two processes in EDA. 
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  5. A detailed chemical understanding of H2 interactions with binding sites in the nanoporous crystalline structure of metal–organic frameworks (MOFs) can lay a sound basis for the design of new sorbent materials. Computational quantum chemical calculations can aid in this quest. To set the stage, we review general thermodynamic considerations that control the usable storage capacity of a sor- bent. We then discuss cluster modeling of H2 ligation at MOF binding sites using state-of-the-art density functional theory (DFT) calculations, and how the binding can be understood using energy decomposition analysis (EDA). Employing these tools, we illustrate the connections between the character of the MOF binding site and the associated adsorption thermodynamics using four experi- mentally characterized MOFs, highlighting the role of open metal sites (OMSs) in accessing binding strengths relevant to room temperature storage. The sorbents are MOF-5, with no open metal sites, Ni2(m-dobdc), containing Lewis acidic Ni(II) sites, Cu(I)-MFU-4l, containing π basic Cu(I) sites and V2Cl2.8(btdd), also containing π-basic V(II) sites. We next explore the potential for binding multiple H2 molecules at a single metal site, with thermodynamics useful for storage at ambient temperature; a materials design goal which has not yet been experimentally demonstrated. Computations on Ca2+ or Mg2+ bound to catecholate or Ca2+ bound to porphyrin show the potential for binding up to 4 H2; there is precedent for the inclusion of both catecholate and porphyrin motifs in MOFs. Turning to transition metals, we discuss the prediction that two H2 molecules can bind at V(II)-MFU-4l, a material that has been synthesized with solvent coordinated to the V(II) site. Additional calculations demonstrate binding three equivalents of hydrogen per OMS in Sc(I) or Ti(I)-exchanged MFU-4l. Overall, the results suggest promising prospects for experimentally realizing higher capacity hydrogen storage MOFs, if nontrivial synthetic and desolvation challenges can be overcome. Coupled with the unbounded chemical diversity of MOFs, there is ample scope for additional exploration and discovery. 
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    Free, publicly-accessible full text available February 22, 2025
  6. The quantum chemistry community has developed analytic forces for approximate electronic excited states to enable walking on excited state potential energy surfaces (PES). One can thereby computationally characterize excited state minima and saddle points. Always implicit in using this machinery is the fact that an excited state PES only exists within the realm of the Born-Oppenheimer approximation, where the nuclear and electronic degrees of freedom separate. This work demonstrates through ab initio calculations and simple nonadiabatic dynamics that some excited state minimum structures are fantastical: they appear to exist as stable configurations only as a consequence of the PES construct, rather than being physically observable. Each fantastical structure exhibits an unphysically high predicted harmonic frequency and associated force constant. This fact can serve as a valuable diagnostic of when an optimized excited state structure is non-observable. The origin of this phenomenon can be attributed to the coupling between different electronic states. As PESs approach one another, the upper surface can form a minimum that is very close to a near-touching point. The force constant, evaluated at this minimum, relates to the strength of the electronic coupling rather than to any characteristic excited state vibration. Nonadiabatic dynamics results using a Landau-Zener model illustrate that fantastical excited state structures have extremely short lifetimes on the order of a few femtoseconds. Their appearance in a calculation signals the presence of a nearby conical intersection through which the system will rapidly cross to a lower surface.

     
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