A new electron-methanol molecule pseudopotential and its application for the solvated electron in methanol

A new electron–methanol molecule pseudopotential is developed and tested in the present paper. The formal development of the potential is based on quantum mechanical calculations on the electron-methanol molecule model in the static exchange approximation. The computational model includes a steep confining potential that keeps the otherwise unbound excess electron in the vicinity of the methanol molecule. Using the Phillips-Kleinman theorem we introduce a smooth pseudo-wave function of the excess electron with the exact eigenenergy and correct asymptotic behavior. The non-local potential energy operator of the model Hamiltonian is then replaced to a local potential that reproduces the ground-state properties of the excess electron satisfactorily. The pseudopotential is then optimized in an analytically simple functional form to fit this approximate local potential in conjunction with the point charges and the geometry of a classical, all-site methanol-methanol interaction potential. Of the adjustable parameters, the parameters for the carbon and the methyl hydrogen atoms are optimized, while those for the oxygen and the hydroxyl hydrogen are taken from a previous electron-water molecule pseudopotential. A polarization term is added to the potential a posteriori. The polarization parameters are chosen to reproduce the experimental position of the optical absorption spectrum of an excess electron in mixed quantum-classical molecular dynamics simulations. The energetic, structural and spectroscopic properties of the solvated electron in a methanol bath are simulated at 300 K, and compared to previous solvated electron simulations and available experimental data

Quantum-Classical Simulation of Electron Localization in Negatively Charged Methanol Clusters

A series of quantum molecular dynamics simulations have been performed to investigate the energetic, structural, dynamic and spectroscopic properties of methanol cluster anions, [(CH3OH)n]– , (n = 50 – 500). Consistent with the inference from photo-electron imaging experiments, we find two main localization modes of the excess electron in equilibrated methanol clusters at ~200 K. The two different localization patterns have strikingly different physical properties, consistent with experimental observations, and are manifest in comparable cluster sizes to those observed. Smaller clusters (n≤128) tend to localize the electron in very weakly bound, diffuse electronic states on the surface of the cluster, while in larger ones the electron is stabilized in solvent cavities, in compact interior-bound states. The interior states exhibit properties that largely resemble and smoothly extrapolate to those simulated for a solvated electron in bulk methanol. The surface electronic states of methanol cluster anions are significantly more weakly bound than the surface states of the anionic water clusters. The key source of the difference is the lack of stabilizing free hydroxyl groups on a relaxed methanol cluster surface. We also provide a mechanistic picture that illustrates the essential role of the interactions of the excess electron with the hydroxyl groups in the dynamic process of excess electron transition from surface-bound states to interior-bound states

Analysis Of Localization Sites for An Excess Electron In Neutral Methanol Clusters Using Approximate Pseudopotential Quantum-Mechanical Calculations

We have used a recently developed electron–methanol molecule pseudopotential in approximate quantum mechanical calculations to evaluate and statistically analyze the physical properties of an excess electron in the field of equilibrated neutral methanol clusters ((CH3OH)n , n = 50 – 500). The methanol clusters were generated in classical molecular dynamics simulations at nominal 100 K and 200 K temperatures. Topological analysis of the neutral clusters indicates that methyl groups cover the surface of the clusters almost exclusively, while the associated hydroxyl groups point inside. Since the initial neutral clusters are lacking polarity on the surface and compact inside, the excess electron can barely attach to these structures. Nevertheless, most of the investigated cluster configurations do support weakly stabilized cluster anion states. We find that similarly to water clusters, the pre-existing instantaneous dipole moment of the neutral clusters binds the electron. The localizing electrons occupy diffuse, weakly bound surface states that largely engulf the cluster although their centers are located outside the cluster molecular frame. The initial localization of the excess electron is reflected in its larger radius compared to water due to the lack of free OH hydrogens on the cluster surface. The stabilization of the excess electron increases, while the radius decreases monotonically as the clusters grow in size. Stable, interior bound states of the excess electron are not observed to form neither in finite size methanol clusters nor in the equilibrium bulk

Preconditioners for the geometry optimisation and saddle point search of molecular systems

A class of preconditioners is introduced to enhance geometry optimisation and transition state search of molecular systems. We start from the Hessian of molecular mechanical terms, decompose it and retain only its positive definite part to construct a sparse preconditioner matrix. The construction requires only the computation of the gradient of the corresponding molecular mechanical terms that are already available in popular force field software packages. For molecular crystals, the preconditioner can be combined straightforwardly with the exponential preconditioner recently introduced for periodic systems. The efficiency is demonstrated on several systems using empirical, semiempirical and ab initio potential energy surfaces

Excess electrons in methanol clusters: beyond the one-electron picture

We performed a series of comparative quantum chemical calculations on various size negatively charged methanol clusters, (CH3OH)n- . The clusters are examined in their optimized geometries (n = 2 - 4), and in geometries taken from mixed quantum-classical molecular dynamics simulations at finite temperature (n = 2 - 128). These latter structures model potential electron binding sites in methanol clusters and in bulk methanol. In particular, we compute the vertical detachment energy (VDE) of an excess electron from increasing size methanol cluster anions using quantum chemical computations at various level of theory including a one-electron pseudopotential model, several density functional theory (DFT) based methods, MP2 and CCSD(T) calculations. The results suggest that at least four methanol molecules are needed to bind an excess electron on a hydrogen bonded methanol chain in a dipole bound state. Larger methanol clusters are able to form stronger interactions with an excess electron. The two simulated excess electron binding motifs in methanol clusters, interior and surface states, correlate well with distinct, experimentally found VDE tendencies with size. Interior states in a solvent cavity are stabilized significantly stronger than electron states on cluster surfaces. Although we find that all the examined quantum chemistry methods more or less overestimate the strength of the experimental excess electron stabilization, MP2, LC-BLYP and BHandHLYP methods with diffuse basis sets provide a significantly better estimate of the VDE than traditional DFT methods (BLYP, B3LYP, X3LYP, PBE0). A comparison to the better performing many electron methods indicates that the examined one-electron pseudopotential can be reasonably used in simulations for systems of larger size

Ab Initio Molecular Dynamics Study of Solvated Electrons in Methanol Clusters

We performed a series of ab initio molecular dynamics simulations to investigate the physical properties of small methanol cluster anions, [(CH3OH)n]–, (n = 8 – 32). The excess electron was attached to neutral clusters that were prepared to accommodate the electron in interior cavity states or surface bound states. The computed initial binding energies of the electrons to these clusters indicate appealing similarity to the experimentally observed vertical detachment energies. The tendency of the interior state clusters parallels that of the clusters with strong electron binding in the experiments, while the simulated unrelaxed surface state anions are similar to the observed weakly bound species. This assignment is consistent with a previous identification based on hybrid quantum-classical simulations. The time evolution of the cluster anions suggest that interior state electrons slowly move to and relax on the surface, in excess electronic states that appear significantly more stable than the experimentally assigned putative surface states. Based on this result we predict the existence of relaxed surface state isomers of small methanol cluster anions. Due to the kinetic metastability of the experimentally found weakly bound species, we anticipate a serious technical challenge to prepare and identify small methanol cluster anions with relaxed surface states. These more strongly binding surface states are stabilized by dangling hydroxyl hydrogen atoms pointing to the excess electron’s charge distribution. In addition, methyl hydrogens also appear to contribute to the stability of these states. During its transition to the surface, the interior excess electron maintains its initial solvent cavity. No signs of non-cavity interior states are observed in the present first principle ab initio molecular dynamics simulations

Exploration, Sampling, And Reconstruction of Free Energy Surfaces with Gaussian Process Regression.

Practical free energy reconstruction algorithms involve three separate tasks: biasing, measuring some observable, and finally reconstructing the free energy surface from those measurements. In more than one dimension, adaptive schemes make it possible to explore only relatively low lying regions of the landscape by progressively building up the bias toward the negative of the free energy surface so that free energy barriers are eliminated. Most schemes use the final bias as their best estimate of the free energy surface. We show that large gains in computational efficiency, as measured by the reduction of time to solution, can be obtained by separating the bias used for dynamics from the final free energy reconstruction itself. We find that biasing with metadynamics, measuring a free energy gradient estimator, and reconstructing using Gaussian process regression can give an order of magnitude reduction in computational cost