Ben Slater - Academia.edu (original) (raw)
Papers by Ben Slater
Cornell University - arXiv, Apr 11, 2019
Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Ty... more Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1-5 K. This low energy scale poses severe challenges for experimental studies of spinices and the practical exploitation of their unusual properties. Here, we show 1
The Journal of Physical Chemistry C, 2016
The Journal of Physical Chemistry C, 2015
Molecular Simulation, 2012
Ab initio molecular dynamics (AIMD) based on density functional theory has been used to study sma... more Ab initio molecular dynamics (AIMD) based on density functional theory has been used to study small aluminium–oxygen complexes in water. Such Al–O clusters have been seen in several recent mass spectrometry studies. In this study, we have focused on trimeric Al–O clusters. The initial very compact trimeric Al–O structures opened up and formed linear Al–O chains. The typical Al–O coordination
Phys Chem Chem Phys, 2001
Page 1. Structure of the surfaces of calcite, dolomite and magnesite (10164) under wet and dry co... more Page 1. Structure of the surfaces of calcite, dolomite and magnesite (10164) under wet and dry conditions Kate Wright,*§ac Randall T. Cyganb and Ben Slaterc a Department of Earth Sciences, University of Manchester, Oxford ...
Pccp Physical Chemistry Chemical Physics, Nov 29, 2005
This study investigates the behaviour of selected, morphologically important surfaces of dolomite... more This study investigates the behaviour of selected, morphologically important surfaces of dolomite (CaMg(CO3)2), using computational modelling techniques. Interatomic potential methods have been used to examine impurity substitution at cationic sites in these surfaces. Environmentally prevalent cations were studied to this end, namely Ni2+, Co2+, Zn2+, Fe2+, Mn2+ and Cd2+, all of which are also found as end-member carbonate minerals. Solid-solution substitution was investigated and showed that Cd and Mn will substitute from their end-member carbonate phase at either dolomite cation site. Mn is found to preferentially substitute at Mg sites, in agreement with experimental findings. For Ni2+, Co2+ and Zn2+, the magnitude of substitution energies is approximately equal for all surfaces, with the exception of the (1014) surface. However, for the larger cations, a far greater disparity in substitution energies is observed. At a stepped surface, analogous substitutions were performed and it was found that substitution energies for all impurity cations were reduced, indicating that uptake is more viable during growth. The predominant surface, the (1014), was solvated with a monolayer of water in order to investigate the influence of hydration on substitution energetics. The addition of water changes the relative preference for substitution of the different cations. Under aqueous conditions, the substitution energy is determined by three competing factors, the relative importance of which cannot be predicted without this type of computational investigation.
Angewandte Chemie, 2014
The synthesis of metal-organic frameworks with large three-dimensional channels that are permanen... more The synthesis of metal-organic frameworks with large three-dimensional channels that are permanently porous and chemically stable offers new opportunities in areas such as catalysis and separation. Two linkers (L1 = 4,4',4'',4'''-([1,1'biphenyl]-3,3',5,5'-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoic acid, L2 = 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid) were used that have equivalent connectivity and dimensions but quite distinct torsional flexibility. With these, a solid solution material, [Zr 6 O 4 (OH) 4 (L1) 2.6 (L2) 0.4 ]·(solvent) x , was formed that has three-dimensional crystalline permanent porosity with a surface area of over 4000 m 2 g À1 that persists after immersion in water. These properties are not accessible for the isostructural phases made from the separate single linkers.
The Journal of Chemical Physics, Jul 21, 2009
The phase diagram of water at negative pressures as obtained from computer simulations for two mo... more The phase diagram of water at negative pressures as obtained from computer simulations for two models of water, TIP4P/2005 and TIP5P is presented. Several solid structures with lower densities than ice Ih, so-called virtual ices, were considered as possible candidates to occupy the negative pressure region of the phase diagram of water. In particular the empty hydrate structures sI, sII, and sH and another, recently proposed, low-density ice structure. The relative stabilities of these structures at 0 K was determined using empirical water potentials and density functional theory calculations. By performing free energy calculations and Gibbs-Duhem integration the phase diagram of TIP4P/2005 was determined at negative pressures. The empty hydrates sII and sH appear to be the stable solid phases of water at negative pressures. The phase boundary between ice Ih and sII clathrate occurs at moderate negative pressures, while at large negative pressures sH becomes the most stable phase. This behavior is in reasonable agreement with what is observed in density functional theory calculations.
Journal of the American Chemical Society, Jan 29, 2012
Combining long-range magnetic order with polarity in the same structure is a prerequisite for the... more Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO(3) (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO(3) has a weak ferromagnetic ground state below 356 K-this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO(3).
The Journal of Chemical Physics, Jul 12, 2013
Density-functional theory (DFT) has been widely used to study water and ice for at least 20 years... more Density-functional theory (DFT) has been widely used to study water and ice for at least 20 years. However, the reliability of different DFT exchange-correlation (xc) functionals for water remains a matter of considerable debate. This is particularly true in light of the recent development of DFT based methods that account for van der Waals (vdW) dispersion forces. Here, we report a detailed study with several xc functionals (semi-local, hybrid, and vdW inclusive approaches) on ice Ih and six proton ordered phases of ice. Consistent with our previous study [Phys. Rev. Lett. 107, 185701 (2011)] which showed that vdW forces become increasingly important at high pressures, we find here that all vdW inclusive methods considered improve the relative energies and transition pressures of the high-pressure ice phases compared to those obtained with semi-local or hybrid xc functionals. However, we also find that significant discrepancies between experiment and the vdW inclusive approaches remain in the cohesive properties of the various phases, causing certain phases to be absent from the phase diagram. Therefore, room for improvement in the description of water at ambient and high pressures remains and we suggest that because of the stern test the high pressure ice phases pose they should be used in future benchmark studies of simulation methods for water.
Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice n... more Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for an FCC (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation, and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.
Physical Review Letters, Nov 1, 2008
Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disor... more Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disorder of the hydrogen atoms in the hydrogen bonded network of the lattice. Here we arrive at a first principles determination of the surface energy of ice Ih and suggest that the surface of ice is significantly more proton ordered than the bulk. We predict that the proton order-disorder transition, which occurs in the bulk at ˜72K, will not occur at the surface at any temperature below surface melting. An order parameter which defines the surface energy of ice Ih surfaces is also identified.
The Journal of chemical physics, Jan 28, 2015
The formation, chemical, and thermal processing of complex organic molecules (COMs) is currently ... more The formation, chemical, and thermal processing of complex organic molecules (COMs) is currently a topic of much interest in interstellar chemistry. The isomers glycolaldehyde, methyl formate, and acetic acid are particularly important because of their role as pre-biotic species. It is becoming increasingly clear that many COMs are formed within interstellar ices which are dominated by water. Hence, the interaction of these species with water ice is crucially important in dictating their behaviour. Here, we present the first detailed comparative study of the adsorption and thermal processing of glycolaldehyde, methyl formate, and acetic acid adsorbed on and in water ices at astrophysically relevant temperatures (20 K). We show that the functional group of the isomer dictates the strength of interaction with water ice, and hence the resulting desorption and trapping behaviour. Furthermore, the strength of this interaction directly affects the crystallization of water, which in turn a...
Ice formation is one of the most common and important processes on Earth and almost always occurs... more Ice formation is one of the most common and important processes on Earth and almost always occurs at the surface of a material. A basic understanding of how the physiochemical properties of a material's surface affects its ability to form ice has remained elusive. Here we use molecular dynamics simulations to directly probe heterogeneous ice nucleation at an hexagonal surface of a nanoparticle of varying hydrophilicity. Surprisingly, we find that structurally identical surfaces can both inhibit and promote ice formation and analogous to a chemical catalyst, it is found that an optimal interaction between the surface and the water exists for promoting ice nucleation. We use our microscopic understanding of the mechanism to design a modified surface in silico with enhanced ice nucleating ability.
In this document we provide brief descriptions of the ice structures and details of the simulatio... more In this document we provide brief descriptions of the ice structures and details of the simulations with DFT, vdW corrected DFT, and DMC. Details of additional calculations done to ensure the accuracy of the results in the main manuscript are reported. We also report results illustrating the sensitivity of the lattice energies of certain ice phases to the percentage of Hartree-Fock exchange used in the hybrid DFT calculations. * angelos.michaelides@ucl.ac.uk
The Journal of chemical physics, Jan 14, 2015
Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice n... more Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for a face centered cubic (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.
The Journal of Physical Chemistry A, 2015
We present the first detailed comparative study of the adsorption and thermal processing of the t... more We present the first detailed comparative study of the adsorption and thermal processing of the three astrophysically important C2O2H4 isomers glycolaldehyde, methyl formate and acetic acid adsorbed on a graphitic grain analogue at 20 K. The ability of the individual molecule to form intermolecular hydrogen bonds is extremely important, dictating the growth modes of the ice on the surface and the measured desorption energies. Methyl formate forms only weak intermolecular bonds and hence wets the graphite surface, forming monolayer, bilayer and multilayer ices, with the multilayer having a desorption energy of 35 kJ mol-1. In contrast, glycolaldehyde and acetic acid dewet the surface, forming clusters even at the very lowest coverages. The strength of the intermolecular hydrogen bonding for glycolaldehyde and acetic acid is reflected in their desorption energies (46.8 and 55 kJ mol-1 respectively), which are comparable to those measured for other hydrogen bonded species such as water. Infrared spectra show that all three isomers undergo structural changes as a result of thermal processing. In the case of acetic acid and glycolaldehyde, this can be assigned to the formation of well-ordered, crystalline, structures where the molecules form chains of hydrogen bonded moieties. The data reported here are of relevance to astrochemical studies of hot cores and star forming regions and can be used to model desorption from interstellar ices during the warm up phase with particular importance for complex organic molecules.
Studies in Surface Science and Catalysis, 2007
We present an investigation of the role of organic templates in the post-nucleation crystal growt... more We present an investigation of the role of organic templates in the post-nucleation crystal growth of nanoporous materials. Although templates are widely used to direct the synthesis towards a desired structure, we have endeavoured to investigate how these templates additionally influence the morphology of synthesised crystals. Recently developed computational approaches have allowed us to examine the role of tetrapropylammonium in
Cornell University - arXiv, Apr 11, 2019
Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Ty... more Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1-5 K. This low energy scale poses severe challenges for experimental studies of spinices and the practical exploitation of their unusual properties. Here, we show 1
The Journal of Physical Chemistry C, 2016
The Journal of Physical Chemistry C, 2015
Molecular Simulation, 2012
Ab initio molecular dynamics (AIMD) based on density functional theory has been used to study sma... more Ab initio molecular dynamics (AIMD) based on density functional theory has been used to study small aluminium–oxygen complexes in water. Such Al–O clusters have been seen in several recent mass spectrometry studies. In this study, we have focused on trimeric Al–O clusters. The initial very compact trimeric Al–O structures opened up and formed linear Al–O chains. The typical Al–O coordination
Phys Chem Chem Phys, 2001
Page 1. Structure of the surfaces of calcite, dolomite and magnesite (10164) under wet and dry co... more Page 1. Structure of the surfaces of calcite, dolomite and magnesite (10164) under wet and dry conditions Kate Wright,*§ac Randall T. Cyganb and Ben Slaterc a Department of Earth Sciences, University of Manchester, Oxford ...
Pccp Physical Chemistry Chemical Physics, Nov 29, 2005
This study investigates the behaviour of selected, morphologically important surfaces of dolomite... more This study investigates the behaviour of selected, morphologically important surfaces of dolomite (CaMg(CO3)2), using computational modelling techniques. Interatomic potential methods have been used to examine impurity substitution at cationic sites in these surfaces. Environmentally prevalent cations were studied to this end, namely Ni2+, Co2+, Zn2+, Fe2+, Mn2+ and Cd2+, all of which are also found as end-member carbonate minerals. Solid-solution substitution was investigated and showed that Cd and Mn will substitute from their end-member carbonate phase at either dolomite cation site. Mn is found to preferentially substitute at Mg sites, in agreement with experimental findings. For Ni2+, Co2+ and Zn2+, the magnitude of substitution energies is approximately equal for all surfaces, with the exception of the (1014) surface. However, for the larger cations, a far greater disparity in substitution energies is observed. At a stepped surface, analogous substitutions were performed and it was found that substitution energies for all impurity cations were reduced, indicating that uptake is more viable during growth. The predominant surface, the (1014), was solvated with a monolayer of water in order to investigate the influence of hydration on substitution energetics. The addition of water changes the relative preference for substitution of the different cations. Under aqueous conditions, the substitution energy is determined by three competing factors, the relative importance of which cannot be predicted without this type of computational investigation.
Angewandte Chemie, 2014
The synthesis of metal-organic frameworks with large three-dimensional channels that are permanen... more The synthesis of metal-organic frameworks with large three-dimensional channels that are permanently porous and chemically stable offers new opportunities in areas such as catalysis and separation. Two linkers (L1 = 4,4',4'',4'''-([1,1'biphenyl]-3,3',5,5'-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoic acid, L2 = 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid) were used that have equivalent connectivity and dimensions but quite distinct torsional flexibility. With these, a solid solution material, [Zr 6 O 4 (OH) 4 (L1) 2.6 (L2) 0.4 ]·(solvent) x , was formed that has three-dimensional crystalline permanent porosity with a surface area of over 4000 m 2 g À1 that persists after immersion in water. These properties are not accessible for the isostructural phases made from the separate single linkers.
The Journal of Chemical Physics, Jul 21, 2009
The phase diagram of water at negative pressures as obtained from computer simulations for two mo... more The phase diagram of water at negative pressures as obtained from computer simulations for two models of water, TIP4P/2005 and TIP5P is presented. Several solid structures with lower densities than ice Ih, so-called virtual ices, were considered as possible candidates to occupy the negative pressure region of the phase diagram of water. In particular the empty hydrate structures sI, sII, and sH and another, recently proposed, low-density ice structure. The relative stabilities of these structures at 0 K was determined using empirical water potentials and density functional theory calculations. By performing free energy calculations and Gibbs-Duhem integration the phase diagram of TIP4P/2005 was determined at negative pressures. The empty hydrates sII and sH appear to be the stable solid phases of water at negative pressures. The phase boundary between ice Ih and sII clathrate occurs at moderate negative pressures, while at large negative pressures sH becomes the most stable phase. This behavior is in reasonable agreement with what is observed in density functional theory calculations.
Journal of the American Chemical Society, Jan 29, 2012
Combining long-range magnetic order with polarity in the same structure is a prerequisite for the... more Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO(3) (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO(3) has a weak ferromagnetic ground state below 356 K-this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO(3).
The Journal of Chemical Physics, Jul 12, 2013
Density-functional theory (DFT) has been widely used to study water and ice for at least 20 years... more Density-functional theory (DFT) has been widely used to study water and ice for at least 20 years. However, the reliability of different DFT exchange-correlation (xc) functionals for water remains a matter of considerable debate. This is particularly true in light of the recent development of DFT based methods that account for van der Waals (vdW) dispersion forces. Here, we report a detailed study with several xc functionals (semi-local, hybrid, and vdW inclusive approaches) on ice Ih and six proton ordered phases of ice. Consistent with our previous study [Phys. Rev. Lett. 107, 185701 (2011)] which showed that vdW forces become increasingly important at high pressures, we find here that all vdW inclusive methods considered improve the relative energies and transition pressures of the high-pressure ice phases compared to those obtained with semi-local or hybrid xc functionals. However, we also find that significant discrepancies between experiment and the vdW inclusive approaches remain in the cohesive properties of the various phases, causing certain phases to be absent from the phase diagram. Therefore, room for improvement in the description of water at ambient and high pressures remains and we suggest that because of the stern test the high pressure ice phases pose they should be used in future benchmark studies of simulation methods for water.
Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice n... more Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for an FCC (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation, and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.
Physical Review Letters, Nov 1, 2008
Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disor... more Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disorder of the hydrogen atoms in the hydrogen bonded network of the lattice. Here we arrive at a first principles determination of the surface energy of ice Ih and suggest that the surface of ice is significantly more proton ordered than the bulk. We predict that the proton order-disorder transition, which occurs in the bulk at ˜72K, will not occur at the surface at any temperature below surface melting. An order parameter which defines the surface energy of ice Ih surfaces is also identified.
The Journal of chemical physics, Jan 28, 2015
The formation, chemical, and thermal processing of complex organic molecules (COMs) is currently ... more The formation, chemical, and thermal processing of complex organic molecules (COMs) is currently a topic of much interest in interstellar chemistry. The isomers glycolaldehyde, methyl formate, and acetic acid are particularly important because of their role as pre-biotic species. It is becoming increasingly clear that many COMs are formed within interstellar ices which are dominated by water. Hence, the interaction of these species with water ice is crucially important in dictating their behaviour. Here, we present the first detailed comparative study of the adsorption and thermal processing of glycolaldehyde, methyl formate, and acetic acid adsorbed on and in water ices at astrophysically relevant temperatures (20 K). We show that the functional group of the isomer dictates the strength of interaction with water ice, and hence the resulting desorption and trapping behaviour. Furthermore, the strength of this interaction directly affects the crystallization of water, which in turn a...
Ice formation is one of the most common and important processes on Earth and almost always occurs... more Ice formation is one of the most common and important processes on Earth and almost always occurs at the surface of a material. A basic understanding of how the physiochemical properties of a material's surface affects its ability to form ice has remained elusive. Here we use molecular dynamics simulations to directly probe heterogeneous ice nucleation at an hexagonal surface of a nanoparticle of varying hydrophilicity. Surprisingly, we find that structurally identical surfaces can both inhibit and promote ice formation and analogous to a chemical catalyst, it is found that an optimal interaction between the surface and the water exists for promoting ice nucleation. We use our microscopic understanding of the mechanism to design a modified surface in silico with enhanced ice nucleating ability.
In this document we provide brief descriptions of the ice structures and details of the simulatio... more In this document we provide brief descriptions of the ice structures and details of the simulations with DFT, vdW corrected DFT, and DMC. Details of additional calculations done to ensure the accuracy of the results in the main manuscript are reported. We also report results illustrating the sensitivity of the lattice energies of certain ice phases to the percentage of Hartree-Fock exchange used in the hybrid DFT calculations. * angelos.michaelides@ucl.ac.uk
The Journal of chemical physics, Jan 14, 2015
Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice n... more Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for a face centered cubic (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.
The Journal of Physical Chemistry A, 2015
We present the first detailed comparative study of the adsorption and thermal processing of the t... more We present the first detailed comparative study of the adsorption and thermal processing of the three astrophysically important C2O2H4 isomers glycolaldehyde, methyl formate and acetic acid adsorbed on a graphitic grain analogue at 20 K. The ability of the individual molecule to form intermolecular hydrogen bonds is extremely important, dictating the growth modes of the ice on the surface and the measured desorption energies. Methyl formate forms only weak intermolecular bonds and hence wets the graphite surface, forming monolayer, bilayer and multilayer ices, with the multilayer having a desorption energy of 35 kJ mol-1. In contrast, glycolaldehyde and acetic acid dewet the surface, forming clusters even at the very lowest coverages. The strength of the intermolecular hydrogen bonding for glycolaldehyde and acetic acid is reflected in their desorption energies (46.8 and 55 kJ mol-1 respectively), which are comparable to those measured for other hydrogen bonded species such as water. Infrared spectra show that all three isomers undergo structural changes as a result of thermal processing. In the case of acetic acid and glycolaldehyde, this can be assigned to the formation of well-ordered, crystalline, structures where the molecules form chains of hydrogen bonded moieties. The data reported here are of relevance to astrochemical studies of hot cores and star forming regions and can be used to model desorption from interstellar ices during the warm up phase with particular importance for complex organic molecules.
Studies in Surface Science and Catalysis, 2007
We present an investigation of the role of organic templates in the post-nucleation crystal growt... more We present an investigation of the role of organic templates in the post-nucleation crystal growth of nanoporous materials. Although templates are widely used to direct the synthesis towards a desired structure, we have endeavoured to investigate how these templates additionally influence the morphology of synthesised crystals. Recently developed computational approaches have allowed us to examine the role of tetrapropylammonium in