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On the effect of solute-solvent Pauli repulsion on n → π* transition for acrolein in water solution
Full text linkIn this work, we present a method to compute the Pauli repulsion contribution to the solute-solvent interaction that exploits solute electronic configurations sampled by Quantum Monte Carlo simulations. Starting from the inspiring model of Amovilli and Mennucci, the discreteness of the solvent is recovered by the definition of molecular domains and the concept of probe molecule. The method can be calibrated on the solute ground state but it offers the advantage of being able to be applied also to electronic-excited states. We show the results for the formaldehyde-water intermolecular pair, here used for the calibration, and two clusters containing acrolein surrounded by 11 and 19 water molecules simulating the solvation shell. In these systems, hydrogen bonds are formed between the solute and the water molecules and we found that, in such case, the Pauli repulsion contribution gives a red shift in the n → π* vertical transition energy
Intermolecular Pauli repulsion: a QMC study of molecules in ground and excited state in free space and in solution
Full text linkIn this work we present a method to compute the Pauli repulsion interaction energy between two molecules and for a molecule solvated by a discrete medium. The method of Amovilli and Mennucci, that has been developed within a continuum solvent model approach, is here revised in order to treat the solvation environment with a discrete number of solvent molecules. In our model, one of the two interacting systems, and the solvent in the case of solvation, acts as ‘probe’. A probe has a volume domain defined by the atomic spheres centred on the nuclei of the relevant molecule. The probe measures the fraction of electrons of the solute molecule falling in its domain leading to the evaluation of Pauli repulsion energy. To this end, Quantum Monte Carlo calculations are used to sample the electronic configurations of the solute. The method has been designed to be applied also to excited states. We show results for test systems in the ground state and for the ground and the (Formula presented.) excited states of acetone in a cluster with 14 water molecules
Ionic hydration at ambient and higher pressures: Computed chemical potentials from simulations and finite-size effects
No full textFor the chemical potential of a hydrated mono-atomic ion,
finite- size effects on simulation results obtained using molecular potential truncation are investigated. Free energy perturbation (FEP) calculations
were carried out by scaling in two processes the Lennard-Jones (LJ) ion-water parameters and the ion-charge (q) for Br-, K+ and Ca2+ interacting with TIP4P water. Corrections which scale with q2 enable us to reduce finite-size effects.
However, at ambient conditions, discrepancies which depend on q are shown by the corrected values when comparison is made with the experimental data
of the Marcus compilation. Similar behavior was observed
by extrapolating the original FEP results to an infinitely large system.
Hence, these errors were assumed to depend on water density and corrected at high pressures. Consistency, within statistical uncertainties, is shown when comparing with results derived from computed volumetric quantities.
Results are also compared with those derived from experimental values of excess volumes at ambient conditions
How increasing pressure affects the ion hydration structure and shell properties at ambient temperature
Full text linkThis work deals with the effect of increasing pressure at 298.15 K on the structure and hydration shell properties of ions in infinitely diluted solutions. Results were obtained from NPT Monte Carlo simulations at various pressures, from 1 atm up to 8000 atm, for some alkali metal, alkaline earth and halide ions in TIP4P water. As pressure increases, the ion-O and the ion-H radial distribution functions (rdfs) are subjected to changes to differing extents depending on both the charge and the size of the ion. The first peak of the ion-O rdf is shifted to a shorter distance from the ion for Cs+, Br− and I−, while in the other cases there is no evidence of shortening. In contrast, for the alkaline earth ions, the most prominent effect is the decrease in the height of the first peak. Minima positions of the ion-O rdfs were used to define the first and the second hydration shells at a given pressure. Water dipole orientation with respect to the radial direction was examined, showing that at higher pressures the ion-dipole interaction becomes less attractive than at 1 atm. A much less favorable orientation was found for waters in the first shell of halide ions. Shell properties were computed from definite integrals of the ion-O rdfs, such as the coordination number and the shell contribution to the excess volume. For the first hydration shell, apart from Ca2+, there is a significant increase in the coordination number upon increasing pressure. This effect becomes more important the larger the ion size is and this is very significant for alkali metal and halide ions. In the case of I− the gap observed between 4 katm and 5 katm reflects the striking effect of increasing pressure on the shell definition. The coordination number remains almost constant when using an alternative boundary for the shell. This was suggested by the radial distribution of water-dipole orientations. Shell excess volume contributions are discussed by examining their dependence on pressure. Electrostriction is shown for the first shell, while the second shell's contribution to the excess volume is positive. At a higher pressure, the shell electrostrictive volume per water molecule is always less than at 1 atm. The greatest effect is shown for the first shell of alkaline earth. The effect of a different shell boundary is examined on the shell quantities of halide ions
Calculation of potential energy surfaces with quantum Monte Carlo as a useful tool for the design of green chemical syntheses: The HOCO radical test case.
No full textShannon Entropy in Atoms: A Test for the Assessment of Density Functionals in Kohn-Sham Theory
Full text linkElectron density is used to compute Shannon entropy. The deviation from the Hartree–Fock (HF) of this quantity has been observed to be related to correlation energy. Thus, Shannon entropy is here proposed as a valid quantity to assess the quality of an energy density functional developed within Kohn–Sham theory. To this purpose, results from eight different functionals, representative of Jacob’s ladder, are compared with accurate results obtained from diffusion quantum Monte Carlo (DMC) computations. For three series of atomic ions, our results show that the revTPSS and the PBE0 functionals are the best, whereas those based on local density approximation give the largest discrepancy from DMC Shannon entropy
Hydrophilic Versus Hydrophobic Coupling in the Pressure Dependence of the Chemical Potential of Alkali Metal and Halide Ions in Water
No full text[Image: see text] We computed the chemical potential for some alkali metal ions (K(+), Rb(+), and Cs(+)) and two halide ions (Br(–) and I(–)) in aqueous solution at ambient T and various pressures in the range 1–8000 atm. Results were obtained from classic Monte Carlo simulations in the NPT ensemble by means of the free energy perturbation method. Here, the chemical potential is computed as the sum of a term relative to a Lennard-Jones solute and a term relative to the process in which this solute is transformed into the ion. Hydrophobic and hydrophilic features of these two components of the chemical potential show opposite behaviors under isothermal compression. The increase in pressure determines an increase in the hydrophobic component, which becomes more positive with a stronger effect for larger ions. Correspondingly, the values of the hydrophilic component become more negative for alkali ions, whereas they are only slightly affected by compression for halide ions. Hydrophobic–hydrophilic quasi-compensation in the slopes is observed for Rb(+). For a smaller ion, such as K(+), the dependence on pressure of the hydrophilic component is slightly dominant. For a larger ion, as observed in the cases of Cs(+), Br(–), and I(–), the hydrophobic component assumes the determinant role. Pressure dependence of the chemical potential is little affected by corrections introduced for molecular potential truncation. This view can change for possible boundary artifacts that could have affected the static electrostatic potential. Some inference is obtained from comparison with experimental data at 1 atm on the free energy of hydration. Discrepancies show the characteristic asymmetry between cations and anions. The further addition of a correction based on the static potential significantly reduces these discrepancies with important error cancellation on the sum of chemical potentials of ions of opposite charge. The correction is applied also at higher pressures, and results are compared with those obtained by adding an alternative correction that is based on the water number density. Regardless of the ion, changes of the chemical potential induced by an increase in pressure appear to be dominated by the hydrophobic component, in particular when using the alternative correction. For bromide and iodide electrolytes, the two corrections give chemical potentials in good agreement
Method to Compute the Solute–Solvent Dispersion Contribution to the Electronic Excitation Energy in Solution
Full text link[Image: see text] A method formulated within the polarizable continuum model of the solvent and a quantum Monte Carlo treatment of the electronic states of the solute molecule is presented for the calculation of the solute–solvent dispersion contribution to the electronic excitation energy in solution. Variational quantum Monte Carlo is exploited to measure the fluctuations of the electronic electric field of the solute molecule to compute the London’s dispersion forces with the solvent. The method previously applied to the ground state of the solute is here extended to excited states. To perform the Casimir–Polder integration, we introduce a positive parameter Ω whose value is properly chosen for this purpose. We derive a general expression that for Ω = 0 reduces to that already proposed for the ground state. For an excited state, Ω must be less than the first transition electronic energy of the solvent molecule but greater than the transition energy from the ground to excited electronic state of the solute molecule. Benchmark calculations were performed on the n → π* transition for formaldehyde, acrolein, and acetone in six solvents, including water, ethanol, cyclohexane, chloroform, carbon tetrachloride, and toluene, and the π → π* transition of acrolein in cyclohexane. Solvents are characterized by their ionization potential and the refractive index at frequency Ω. In all cases, we found that the dispersion solute–solvent interaction stabilizes the excited state of the solutes leading to red (negative) solvatochromic shifts
Shannon entropy and correlation energy for electrons in atoms
No full textIn this work, we compute Shannon entropy, defined in terms of electron density, for three series of atomic ions including the region of nuclear charges close to the limit at which the ionization potential goes to zero.We use both Hartree-Fock (HF) and quantum Monte Carlo (QMC) densities and we observe a sharp positive deviation ofQMCentropy with respect to theHF corresponding value in approaching the limit.We discuss this behaviour taking into account Coulomb correlation, which plays an important role in the weak binding regime
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